[Capture] Allow higher order primitives to accept dynamically shaped arrays

doc/releases/changelog-dev.md

@@ -6,6 +6,9 @@
 
 <h3>Improvements 🛠</h3>
 
+* The higher order primitives in program capture can now accept inputs with abstract shapes.
+  [(#6786)](https://github.com/PennyLaneAI/pennylane/pull/6786)
+
 * The coefficients of observables now have improved differentiability.
   [(#6598)](https://github.com/PennyLaneAI/pennylane/pull/6598)
 

pennylane/capture/__init__.py

@@ -33,6 +33,7 @@
     ~create_measurement_obs_primitive
     ~create_measurement_wires_primitive
     ~create_measurement_mcm_primitive
+    ~determine_abstracted_axes
     ~expand_plxpr_transforms
     ~run_autograph
     ~make_plxpr
@@ -170,6 +171,7 @@ def _(*args, **kwargs):
 )
 from .flatfn import FlatFn
 from .make_plxpr import make_plxpr, run_autograph
+from .dynamic_shapes import determine_abstracted_axes
 
 # by defining this here, we avoid
 # E0611: No name 'AbstractOperator' in module 'pennylane.capture' (no-name-in-module)

pennylane/capture/base_interpreter.py

@@ -385,12 +385,15 @@ def handle_ctrl_transform(self, *invals, n_control, jaxpr, control_values, work_
 
 
 @PlxprInterpreter.register_primitive(for_loop_prim)
-def handle_for_loop(self, start, stop, step, *args, jaxpr_body_fn, consts_slice, args_slice):
+def handle_for_loop(
+    self, start, stop, step, *args, jaxpr_body_fn, consts_slice, args_slice, abstract_shapes_slice
+):
     """Handle a for loop primitive."""
     init_state = args[args_slice]
+    abstract_shapes = args[abstract_shapes_slice]
 
     new_jaxpr_body_fn = jaxpr_to_jaxpr(
-        copy(self), jaxpr_body_fn, args[consts_slice], start, *init_state
+        copy(self), jaxpr_body_fn, args[consts_slice], *abstract_shapes, start, *init_state
     )
 
     return for_loop_prim.bind(
@@ -401,6 +404,7 @@ def handle_for_loop(self, start, stop, step, *args, jaxpr_body_fn, consts_slice,
         jaxpr_body_fn=new_jaxpr_body_fn,
         consts_slice=consts_slice,
         args_slice=args_slice,
+        abstract_shapes_slice=abstract_shapes_slice,
     )
 
 
@@ -424,15 +428,27 @@ def handle_cond(self, *invals, jaxpr_branches, consts_slices, args_slice):
 
 @PlxprInterpreter.register_primitive(while_loop_prim)
 def handle_while_loop(
-    self, *invals, jaxpr_body_fn, jaxpr_cond_fn, body_slice, cond_slice, args_slice
+    self,
+    *invals,
+    jaxpr_body_fn,
+    jaxpr_cond_fn,
+    body_slice,
+    cond_slice,
+    args_slice,
+    abstract_shapes_slice,
 ):
     """Handle a while loop primitive."""
     consts_body = invals[body_slice]
     consts_cond = invals[cond_slice]
     init_state = invals[args_slice]
+    abstract_shapes = invals[abstract_shapes_slice]
 
-    new_jaxpr_body_fn = jaxpr_to_jaxpr(copy(self), jaxpr_body_fn, consts_body, *init_state)
-    new_jaxpr_cond_fn = jaxpr_to_jaxpr(copy(self), jaxpr_cond_fn, consts_cond, *init_state)
+    new_jaxpr_body_fn = jaxpr_to_jaxpr(
+        copy(self), jaxpr_body_fn, consts_body, *abstract_shapes, *init_state
+    )
+    new_jaxpr_cond_fn = jaxpr_to_jaxpr(
+        copy(self), jaxpr_cond_fn, consts_cond, *abstract_shapes, *init_state
+    )
 
     return while_loop_prim.bind(
         *invals,
@@ -441,6 +457,7 @@ def handle_while_loop(
         body_slice=body_slice,
         cond_slice=cond_slice,
         args_slice=args_slice,
+        abstract_shapes_slice=abstract_shapes_slice,
     )
 
 
@@ -482,16 +499,24 @@ def handle_jacobian(self, *invals, jaxpr, n_consts, **params):
 
 
 def flatten_while_loop(
-    self, *invals, jaxpr_body_fn, jaxpr_cond_fn, body_slice, cond_slice, args_slice
+    self,
+    *invals,
+    jaxpr_body_fn,
+    jaxpr_cond_fn,
+    body_slice,
+    cond_slice,
+    args_slice,
+    abstract_shapes_slice,
 ):
     """Handle the while loop by a flattened python strategy."""
     consts_body = invals[body_slice]
     consts_cond = invals[cond_slice]
     init_state = invals[args_slice]
+    abstract_shapes = invals[abstract_shapes_slice]
 
     fn_res = init_state
-    while copy(self).eval(jaxpr_cond_fn, consts_cond, *fn_res)[0]:
-        fn_res = copy(self).eval(jaxpr_body_fn, consts_body, *fn_res)
+    while copy(self).eval(jaxpr_cond_fn, consts_cond, *abstract_shapes, *fn_res)[0]:
+        fn_res = copy(self).eval(jaxpr_body_fn, consts_body, *abstract_shapes, *fn_res)
 
     return fn_res
 
@@ -515,14 +540,17 @@ def flattened_cond(self, *invals, jaxpr_branches, consts_slices, args_slice):
 FlattenedHigherOrderPrimitives[cond_prim] = flattened_cond
 
 
-def flattened_for(self, start, stop, step, *invals, jaxpr_body_fn, consts_slice, args_slice):
+def flattened_for(
+    self, start, stop, step, *invals, jaxpr_body_fn, consts_slice, args_slice, abstract_shapes_slice
+):
     """Handle the for loop by a flattened python strategy."""
     consts = invals[consts_slice]
     init_state = invals[args_slice]
+    abstract_shapes = invals[abstract_shapes_slice]
 
     res = init_state
     for i in range(start, stop, step):
-        res = copy(self).eval(jaxpr_body_fn, consts, i, *res)
+        res = copy(self).eval(jaxpr_body_fn, consts, *abstract_shapes, i, *res)
 
     return res
 

pennylane/capture/dynamic_shapes.py

@@ -0,0 +1,100 @@
+# Copyright 2025 Xanadu Quantum Technologies Inc.
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""
+Contains a utility for handling inputs with dynamically shaped arrays.
+"""
+from string import ascii_lowercase
+
+has_jax = True
+try:
+    import jax
+except ImportError:  # pragma: no cover
+    has_jax = False  # pragma: no cover
+
+
+def _get_shape_for_array(x, abstract_shapes: list) -> dict:
+    """
+    Populate the dictionay of abstract axes for a single tensorlike.
+
+    This dictionary has dimensions as keys, and a string marker as the value.
+
+    Examples of shape -> abstract axes:
+
+    * `(3,4) -> {}`
+    * `(tracer1, ) -> {0: "a"}`
+    * `(tracer1, tracer1) -> {0: "a", 1: "a"}`
+    * `(3, tracer1) -> {1: "a"}`
+    * `(tracer1, 2, tracer2) -> {0: "a", 2: "b"}`
+
+    `abstract_shapes` contains all the tracers found in shapes.
+
+    """
+    abstract_axes = {}
+    for i, s in enumerate(getattr(x, "shape", ())):
+        if not isinstance(s, int):  #  if not int, then abstract
+            found = False
+            # check if the shape tracer is one we have already encountered
+            for previous_idx, previous_shape in enumerate(abstract_shapes):
+                if s is previous_shape:
+                    abstract_axes[i] = ascii_lowercase[previous_idx]
+                    found = True
+                    continue
+            # haven't encountered it, so add it to abstract_axes
+            # and use new letter designation
+            if not found:
+                abstract_axes[i] = ascii_lowercase[len(abstract_shapes)]
+                abstract_shapes.append(s)
+
+    return abstract_axes
+
+
+def determine_abstracted_axes(args):
+    """Computed the abstracted axes and extracing the abstract shapes from the arguments.
+
+    Args:
+        args (tuple): the arguments for a higher order primitive
+
+    Returns:
+        tuple, tuple: the corresponding abstracted axes and dynamic shapes
+
+    See the ``intro_to_dynamic_shapes.md`` document for more information on how dynamic shapes work.
+
+    To make jaxpr from arguments with dynamic shapes, the ``abstracted_axes`` keyword argument must be set.
+    Then, when calling the jaxpr, variables for the dynamic shapes must be passed.
+
+    .. code-block:: python
+
+        def f(n):
+            x = jax.numpy.ones((n,))
+            abstracted_axes, abstract_shapes = qml.capture.determine_abstracted_axes((x,))
+            jaxpr = jax.make_jaxpr(jax.numpy.sum, abstracted_axes=abstracted_axes)(x)
+            return jax.core.eval_jaxpr(jaxpr.jaxpr, jaxpr.consts, *abstract_shapes, x)
+
+    """
+    if not has_jax:  # pragma: no cover
+        raise ImportError("jax must be installed to use determine_abstracted_axes")
+    if not jax.config.jax_dynamic_shapes:  # pylint: disable=no-member
+        return None, tuple()
+
+    args, structure = jax.tree_util.tree_flatten(args)
+
+    abstract_shapes = []
+    # note: this function in-place mutates abstract_shapes
+    # adding any additional abstract shapes found
+    abstracted_axes = [_get_shape_for_array(a, abstract_shapes) for a in args]
+
+    if not abstract_shapes:
+        return None, ()
+    abstracted_axes = jax.tree_util.tree_unflatten(structure, abstracted_axes)
+    return abstracted_axes, abstract_shapes