Update docs for softmax in onnx supported operators (#24832)
Summary:
Update the softmax in onnx supported operators from `softmax (only dim=-1 supported)` to `softmax`, as all cases of dim options are supported in:
[https://github.com/pytorch/pytorch/issues/18482](https://github.com/pytorch/pytorch/pull/18482): ONNX Export All Cases of Softmax
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24832
Differential Revision: D16896538
Pulled By: bddppq
fbshipit-source-id: 284039ffa42f09b0043e95cfe9f17e1afde53814
