[torchlib] Modify aten_unbind to use None for split_sizes

[justinchuby]

According to https://onnx.ai/onnx/operators/onnx__SplitToSequence.html#summary, If the argument split is not specified, a default scalar value of 1 is used as the value of split, and this is the only case when keepdims can be set to 0.

Fixes #2533

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.02%. Comparing base (07f3e4c) to head (fbfee99).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2536      +/-   ##
==========================================
+ Coverage   70.00%   70.02%   +0.01%     
==========================================
  Files         215      215              
  Lines       25992    25991       -1     
  Branches     2606     2606              
==========================================
+ Hits        18196    18200       +4     
+ Misses       6896     6891       -5     
  Partials      900      900              

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[titaiwangms]

I feel like the wording in ONNX spec for mutually exclusive attribute/input is ambiguous: Keep the split dimension or not. Default 1, which means we keep split dimension. If input ‘split’ is specified, this attribute is ignored.