Fix bug in SplitCatSimplifier when next_user is an output node (#103338)
Summary:
When simplifying split cat patterns, if next user of a split node was an output node, there was a bug leading to an issue like: P765993221
Basically, the bug was in how args and kwargs of the user were getting replaced, and the code didn't handle nested arg/kwargs.
Using torch.fx.Node functions such as `all_input_nodes` and `replace_input_with` fixes this issue
Differential Revision: D46603618
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103338
Approved by: https://github.com/jansel
