Fix specailization issue in keyed_jagged_index_select_dim1_forward_cu…

…da (#3578)

Summary:
X-link: facebookresearch/FBGEMM#664


`lengths` is a tensor with symbolic shapes. Calling `len` on it will force specialization on it which will cause data dependent failure as shown below:
 {F1974383976} 

tlparse: https://fburl.com/74rjmr8e

The fix is to replace `len` with equivalent operations which support symbolic shapes.

Differential Revision: D67491452

fbgemm_gpu/fbgemm_gpu/sparse_ops.py

@@ -900,13 +900,14 @@ def keyed_jagged_index_select_dim1_forward_cuda_impl_abstract(
     lengths: torch.Tensor,
     offsets: torch.Tensor,
     indices: torch.Tensor,
-    batch_size: int,
+    batch_size: torch.SymInt,
     weights: Optional[torch.Tensor] = None,
-    selected_lengths_sum: Optional[int] = None,
+    selected_lengths_sum: Optional[torch.SymInt] = None,
 ) -> List[torch.Tensor]:
-    num_batches = len(lengths) // batch_size
-    torch._check(len(lengths) + 1 == len(offsets))
-    torch._check(len(lengths) % batch_size == 0)
+    num_batches = lengths.size(0) // batch_size
+    torch._check(lengths.size(0) + 1 == offsets.size(0))
+    # pyre-ignore
+    torch._check(lengths.size(0) % batch_size == 0)
 
     if weights is not None:
         # weights must have the same shape as values