Excise yaml dependency in torchgen.model (#100203)

The problem:
- The new CustomOp API depends on torchgen.model
- torchgen.model imports `yaml`
- `yaml` is not a PyTorch runtime dependency

To unblock myself, because I'm not sure how long it'll take to
convince people yaml should be a PyTorch runtime dependency
(unless one of you wants to approve #100166), this PR removes the
yaml dependency from torchgen.model.

It does so by splitting torchgen.utils (the offender) into
torchgen.utils (no yaml) and torchgen.yaml (which uses yaml).

Test Plan:
- CI
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100203
Approved by: https://github.com/ezyang, https://github.com/Skylion007

torchgen/utils.py

@@ -28,33 +28,6 @@ from typing import (
 
 from torchgen.code_template import CodeTemplate
 
-# Safely load fast C Yaml loader/dumper if they are available
-try:
-    from yaml import CSafeLoader as Loader
-except ImportError:
-    from yaml import SafeLoader as Loader  # type: ignore[misc]
-
-try:
-    from yaml import CSafeDumper as Dumper
-except ImportError:
-    from yaml import SafeDumper as Dumper  # type: ignore[misc]
-YamlDumper = Dumper
-
-
-# A custom loader for YAML that errors on duplicate keys.
-# This doesn't happen by default: see https://github.com/yaml/pyyaml/issues/165
-class YamlLoader(Loader):
-    def construct_mapping(self, node, deep=False):  # type: ignore[no-untyped-def]
-        mapping = []
-        for key_node, value_node in node.value:
-            key = self.construct_object(key_node, deep=deep)  # type: ignore[no-untyped-call]
-            assert (
-                key not in mapping
-            ), f"Found a duplicate key in the yaml. key={key}, line={node.start_mark.line}"
-            mapping.append(key)
-        mapping = super().construct_mapping(node, deep=deep)  # type: ignore[no-untyped-call]
-        return mapping
-
 
 # Many of these functions share logic for defining both the definition
 # and declaration (for example, the function signature is the same), so