Support for scatter_max

## ❓Question
Hi! I am in the middle of converting a graph neural network which at a point uses a message passing layer and during converting the torch model I get the unsupported operation message for torch.scatter_max operation. I saw that you have a scatter_gather operation with mode max here https://apple.github.io/coremltools/source/coremltools.converters.mil.mil.ops.defs.html#module-coremltools.converters.mil.mil.ops.defs.iOS15.scatter_gather. Is this the same operation as https://pytorch-scatter.readthedocs.io/en/1.3.0/functions/max.html? If yes, how can I reroute it through a custom operation?
Thanks a lot 🙏

Start by comparing the documented scatter_gather operation in the Core ML Tools MIL iOS15 reference with the pytorch-scatter max operation linked in the issue. Reproduce the torch.scatter_max conversion failure, then determine whether equivalence and a supported custom-operation route can be established; done means the conversion path is supported or its limitation is clearly documented.

Written by the indexing model from the issue text.