Support new AIMET format in split onnx utils

[idantene]

In splitting an onnx file with the current code, fill_input_encodings_of_split can fail because it assumes encodings["X_params"] is a dictionary, which is no longer the case with the new AIMET spec for version 1.0.0.
Crucially, this is fixed in the split_onnx_by_names method, but not in the _load_encodings one, so this is a failure point.

[bhushan23]

Hi @idantene thank you for putting up this PR, changes looks great.

We will take this up and merge internally.
Minor change, we can gate this function upon encoding_version so we can support all three versions that we have to make it robust.

[idantene]

Sure, as per the docs I'm only aware of 2 versions (pre-1.0.0 and 1.0.0), so it's interesting to learn you have three versions out.