Consolidate overloads in torchlib

[justinchuby]

The goal is to have no overloads and remove the PyTorch dispatcher. Right now there are still the following ops that need to be addressed:

Registering private function: aten::as_strided
Registering private function: aten::embedding_bag
Registering private function: aten::embedding_bag.padding_idx
Registering overload for function: aten::index.Tensor
Registering overload for function: aten::_unsafe_index.Tensor
Registering overload for function: aten::index_put

I did a bit of cleaning up in tests and torchlib as well.

#2580

[codecov]

Codecov Report

❌ Patch coverage is 79.62963% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.32%. Comparing base (075fc4d) to head (fbc0540).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/function_libs/torch_lib/ops/core.py	80.00%	19 Missing and 12 partials ⚠️
onnxscript/function_libs/torch_lib/ops/nn.py	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2604      +/-   ##
==========================================
+ Coverage   69.97%   70.32%   +0.35%     
==========================================
  Files         222      222              
  Lines       26324    26224     -100     
  Branches     2609     2624      +15     
==========================================
+ Hits        18420    18442      +22     
+ Misses       6992     6865     -127     
- Partials      912      917       +5     

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[justinchuby]

Revert changes to roll