[Torch] Fold no-op reshape (#3769)

This was preventing dynamic dims in an ONNX model from being reified (causing the generation of `tensor.cast`s and preventing fusion in iree):

```mlir
%2 = torch.vtensor.literal(dense<[4, 256]> : tensor<2xsi64>) : !torch.vtensor<[2],si64>]
%7 = torch.prim.ListConstruct %int2 : (!torch.int) -> !torch.list<int>
%8 = torch.aten.reshape %2, %7 : !torch.vtensor<[2],si64>, !torch.list<int> -> !torch.vtensor<[2],si64>
//... chain of foldable ops linking %2 to the `shape` operand of a `torch.aten.broadcast_to ... -> !torch.vtensor<[?,?],si64>`
```

include/torch-mlir/Dialect/Torch/IR/GeneratedTorchOps.td

@@ -11455,6 +11455,7 @@ def Torch_AtenReshapeOp : Torch_Op<"aten.reshape", [
       printDefaultTorchOp(printer, *this, 2, 1);
     }
   }];
+  let hasFolder = 1;
 }
 
 def Torch_AtenReshapeAsOp : Torch_Op<"aten.reshape_as", [

lib/Dialect/Torch/IR/TorchOps.cpp

@@ -2261,6 +2261,19 @@ void AtenUnflattenIntOp::getCanonicalizationPatterns(
   });
 }
 
+//===----------------------------------------------------------------------===//
+// AtenReshapeOp
+//===----------------------------------------------------------------------===//
+
+OpFoldResult AtenReshapeOp::fold(FoldAdaptor adaptor) {
+  auto selfTy = dyn_cast<ValueTensorType>(getSelf().getType());
+  auto opTy = dyn_cast<ValueTensorType>(getType());
+  if (selfTy && selfTy == opTy && selfTy.hasSizes() &&
+      selfTy.toBuiltinTensor().hasStaticShape())
+    return getSelf();
+  return nullptr;
+}
+
 //===----------------------------------------------------------------------===//
 // AtenSelectIntOp
 //===----------------------------------------------------------------------===//

projects/pt1/python/torch_mlir/jit_ir_importer/build_tools/torch_ods_gen.py

@@ -856,7 +856,7 @@ def emit_with_mutating_variants(key, **kwargs):
     emit("aten::repeat : (Tensor, int[]) -> (Tensor)")
     emit("aten::repeat_interleave.self_int : (Tensor, int, int?, int?) -> (Tensor)")
     emit("aten::tile : (Tensor, int[]) -> (Tensor)")
-    emit("aten::reshape : (Tensor, int[]) -> (Tensor)")
+    emit("aten::reshape : (Tensor, int[]) -> (Tensor)", has_folder=True)
     emit("aten::reshape_as : (Tensor, Tensor) -> (Tensor)")
     emit("aten::_reshape_alias : (Tensor, int[], int[]) -> (Tensor)")
     emit("aten::resize : (Tensor, int[], int?) -> (Tensor)")