[LinalgExt] Drop the unit dims on scatter ops

[IanWood1]

This change adds patterns to drop the unit dims of a iree_linalg_ext.scatter's %updates tensor. Changes were also made to the verifier to allow for scatter to not have unit dims. For example, the following would be illegal because the updates tensor has too small of a rank:

%0 = iree_linalg_ext.scatter dimension_map = [0] unique_indices(true)
	ins(%collapsed, %arg1 : tensor<?x16x4x128xf16>, tensor<?x1xi32>) 
	outs(%arg2 : tensor<?x2x16x4x128xf16>) {
    ^bb0(%arg3: f16, %arg4: f16):                                                                                                                                                                 iree_linalg_ext.yield %arg3 : f16
} -> tensor<?x2x16x4x128xf16>

See the related issue #19091