Error while parsing precomputed field for NativeFunctions YAML in torchgen

[apach301]

🐛 Describe the bug

Hi,
I found a bug during testing parse_native_yaml function.

An error occured in Argument parser, but the bug is in missed validation for 'precomputed' field.
Seed to reproduce bug:

- func: index.Tensor_out(Tensor self, Tensor?[] indices, *, Tensor(a!) out) -> Tensor(a!)
  device_check: NoCheck
  structured: True
  structured_inherits: TensorIteratorBase
  precomputed:
  - indices ->dnn_linear(Tensor self, Tensor weight, Tensor? bias=None) -> Tensor
  python_module: nn

Error:

 === Uncaught Python exception: ===
ValueError: not enough values to unpack (expected 2, got 1)
  in /tmp/tmptn11fmou.yaml:3017:
    index.Tensor_out(Tensor self, Tensor?[] indices, *, Tensor(a!) out) -> Tensor(a!)
Traceback (most recent call last):
  File "/parse_native_yaml.py", line 40, in TestOneInput
    parse_native_yaml(filename, "/pytorch/aten/src/ATen/native/tags.yaml")
  File "/usr/local/lib/python3.9/dist-packages/torchgen/gen.py", line 247, in parse_native_yaml
    _GLOBAL_PARSE_NATIVE_YAML_CACHE[path] = parse_native_yaml_struct(
  File "/usr/local/lib/python3.9/dist-packages/torchgen/gen.py", line 169, in parse_native_yaml_struct
    func, m = NativeFunction.from_yaml(e, loc, valid_tags, ignore_keys)
  File "/usr/local/lib/python3.9/dist-packages/torchgen/model.py", line 670, in from_yaml
    precomputed = Precompute.parse(precomputed_dict) if precomputed_dict else None
  File "/usr/local/lib/python3.9/dist-packages/torchgen/model.py", line 2773, in parse
    with_list_args = [Argument.parse(name.strip()) for name in with_list]
  File "/usr/local/lib/python3.9/dist-packages/torchgen/model.py", line 2773, in <listcomp>
    with_list_args = [Argument.parse(name.strip()) for name in with_list]
  File "/usr/local/lib/python3.9/dist-packages/torchgen/model.py", line 1990, in parse
    type_and_annot, name_and_default = arg.rsplit(" ", 1)
ValueError: not enough values to unpack (expected 2, got 1)
  in /tmp/tmptn11fmou.yaml:3017:
    index.Tensor_out(Tensor self, Tensor?[] indices, *, Tensor(a!) out) -> Tensor(a!)

Versions

PyTorch version: 2.3.0, current main

OS: Ubuntu 20.04.6 LTS (x86_64)
GCC version: (Ubuntu 9.4.0-1ubuntu1~20.04.2) 9.4.0
Clang version: 14.0.6
CMake version: version 3.29.2
Libc version: glibc-2.31

Python version: 3.9.5 (default, Nov 23 2021, 15:27:38) [GCC 9.3.0] (64-bit runtime)
Python platform: Linux-5.4.0-174-generic-x86_64-with-glibc2.31

cc @ezyang @bhosmer @bdhirsh @kadeng