Add ability to add MIL ops to ET that are not yet in coremltools (#12648)

This introduces torch_ops.py in ET's CoreML compiler. This lets us add
critical MIL ops that are not yet in ET's version of coremltools.

Author: metascroy

backends/apple/coreml/compiler/coreml_preprocess.py

@@ -28,6 +28,8 @@
 logger = logging.getLogger(__name__)
 logger.setLevel(logging.WARNING)
 
+from executorch.backends.apple.coreml.compiler.torch_ops import *  # noqa: F401, F403
+
 
 class COMPILE_SPEC_KEYS(Enum):
     COMPUTE_UNITS = "compute_units"

backends/apple/coreml/compiler/torch_ops.py

@@ -0,0 +1,26 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+# This file registers torch ops that are not yet in coremltools, or are in a more recent version of
+# coremltools than is used by ExecuTorch.  Each op registered here should have a link to a PR in coremltools that adds
+# the op to the coremltools library.
+
+from coremltools.converters.mil.frontend.torch.ops import transpose, unbind
+from coremltools.converters.mil.frontend.torch.torch_op_registry import (
+    register_torch_op,
+)
+
+
+# https://github.com/apple/coremltools/pull/2556
+@register_torch_op(override=False)
+def transpose_copy(context, node):
+    transpose(context, node)
+
+
+# https://github.com/apple/coremltools/pull/2557
+@register_torch_op(override=False)
+def unbind_copy(context, node):
+    unbind(context, node)