Fix GPTNeo onnx export
# What does this PR do?

#13491 simplified the GPTNeo's local attention by removing the complicated operations, as a result, the `GPTNeoAttentionMixin` class is no longer needed and was removed. This breaks `GPTNeoOnnxConfig` as it was importing the `GPTNeoAttentionMixin` class.

This PR (tries to!) fixes `GPTNeoOnnxConfig` by removing the `PatchingSpec` and updates the `input`, `output` accordingly.

I'm an onnx noob so it would be awesome if you could please take a deeper look @michaelbenayoun :) 

Thanks a lot, @tianleiwu for reporting the bug!
@patil-suraj, thanks for providing a quick fix. 

For dynamic axes mapping for inputs and outputs, I suggest the following:
```
"input_ids": {0: "batch", 1: "sequence"}
f"past_key_values.{i}.key" = {0: "batch", 2: "past_sequence"}
f"present.{i}.key" : {0: "batch", 2: "present_sequence"}      #or {0: "batch", 2: "past_sequence+sequence"}
"attention_mask" : {0: "batch", 1: "present_sequence"}    #or {0: "batch", 1: "past_sequence+sequence"}
```


<|||||>I have updated the onnx config input and output names to support dynamic axes as @tianleiwu suggested.
The simplification brought by #13491 along with this PR also solves the issue #13175.

@sgugger @LysandreJik I have left the custom implementations `custom_unfold` and `custom_get_block_length_and_num_blocks` which are no longer needed thanks to #13491: should we keep them there as they might be useful one day for other models or get rid of them to keep things clean?<|||||>I think we can keep those two functions for now. We can look again in a few months and remove them if they are still not used anywhere.