fix transpose / permutations fusion pass

Summary:
Follow up on D73619452

We noticed that this pattern used to be fused, but stopped being fused after D73619452.

What happens is that we were checking for unitary dimensions. So a tensor of shape [1, 5, 30] is equivalent (in memory) to [5, 1, 30]. So, transposing on [0, 2] then on [0, 1] is (almost) equivalent to an identity and can be fused

This diff re-establishes this behavior.

Differential Revision: D74447383

Author: ThomasJannaud

backends/cadence/aot/fuse_ops.py

@@ -885,6 +885,9 @@ class FuseTransposeOrPermuteOpPairsPass(FuseOpPairsAcrossBranchesPass):
     """
     Fuse transpose or permute op pairs to a single view op.
     (transpose or permutation) -> (quant or dequant) -> (transpose or permutation)
+    This happens when op2(op1) == identity, modulo unitary dimensions.
+    'unitary dimensions' example: a tensor of shape [1, 5, 30] is equivalent (in memory) to [5, 1, 30]
+    so transpose(1, 2) then transpose(0, 2) is a pseudo identity and should be fused.
     """
 
     # A list of ops that can be bypassed when looking for a
@@ -908,7 +911,7 @@ def can_fuse_for_chain(
         if not super().can_fuse_for_chain(producer, consumer, consumer_op_packets):
             return False
 
-        # checking that permut2(permut1(identify)) == identity
+        # checking that permut2(permut1(identify)) == identity, modulo unitary dimensions
         input_shape = cast(torch.fx.Node, producer.args[0]).meta["val"].shape
         ident_dims = list(range(len(input_shape)))
         # this mapping helps to handle both transpose and permutations
@@ -918,14 +921,20 @@ def can_fuse_for_chain(
         }
         in_dims = f[producer.target](producer, ident_dims)
         out_dims = f[consumer.target](consumer, in_dims)
-        return out_dims == ident_dims
+        # Filtering out unitary dimensions
+        non_unit_ident_dims = [dim for dim in ident_dims if input_shape[dim] != 1]
+        non_unit_out_dims = [dim for dim in out_dims if input_shape[dim] != 1]
+        return non_unit_out_dims == non_unit_ident_dims
 
     def get_fused_node(
         self,
         producer: torch.fx.Node,
         consumer: torch.fx.Node,
         graph_module: torch.fx.GraphModule,
     ) -> torch.fx.Node:
+        # This step is important because of how we can fuse transpositions that are not perfectly
+        # reverse one of another but will be fused if there are unitary dimensions.
+        # The fused operation must have the same output shape as the consumer.
         output_shape = consumer.meta["val"].shape
         with graph_module.graph.inserting_after(consumer):
             view = graph_module.graph.call_function(

backends/cadence/aot/tests/test_fusion_ops_passes.py

@@ -584,6 +584,28 @@ def _create_operator(
                 exir_ops.edge.quantized_decomposed.quantize_per_tensor.default,
                 False,
             ),
+            # transpose -> quant -> transpose is not the reverse BUT there is a UNITARY dimension
+            # so it ends up being the same on memory => fuse
+            (
+                True,
+                [0, 1],
+                True,
+                [0, 2],
+                exir_ops.edge.quantized_decomposed.quantize_per_tensor.default,
+                True,
+                [5, 40, 1],
+            ),
+            # transpose -> quant -> transpose is not the reverse, and unitary dimensions
+            # don't help => don't fuse
+            (
+                True,
+                [0, 1],
+                True,
+                [1, 3],
+                exir_ops.edge.quantized_decomposed.quantize_per_tensor.default,
+                False,
+                [5, 40, 1, 4],
+            ),
             # permutation -> quant -> opposite permutation => fuse
             (
                 False,
@@ -622,6 +644,28 @@ def _create_operator(
                 False,
                 [4, 4, 4],
             ),
+            # permutation -> quant -> a non reverse permutation BUT there is a UNITARY dimension
+            # so it ends up being the same on memory => fuse
+            (
+                False,
+                [1, 3, 2, 0],
+                False,
+                [3, 2, 1, 0],
+                exir_ops.edge.quantized_decomposed.quantize_per_tensor.default,
+                True,
+                [3, 1, 8, 10],
+            ),
+            # permutation -> quant -> a non reverse permutation, and unitary dimensions
+            # don't help => don't fuse
+            (
+                False,
+                [1, 3, 2, 0],
+                False,
+                [3, 1, 2, 0],
+                exir_ops.edge.quantized_decomposed.quantize_per_tensor.default,
+                False,
+                [3, 1, 8, 10],
+            ),
             # transpose -> quant -> transpose as a permutation => fuse
             (
                 True,