Add ONNX export support for `PatchTST` (#2101)
* Add ONNX export support for `PatchTST`
* Add unit test for patchtst
* Add listed support for PatchTST
* Add ONNX export support for patchtsmixer
* Add task=feature-extraction
* Fix ONNX compatible unfold
* Formatting
* Correctly handle negative indexing for onnx compatible unfold
* Update tests/exporters/exporters_utils.py
Co-authored-by: Ella Charlaix <80481427+echarlaix@users.noreply.github.com>
* Update optimum/exporters/tasks.py
Co-authored-by: Ella Charlaix <80481427+echarlaix@users.noreply.github.com>
* Move dummy patch tst input generator to input_generators.py
* Code formatting
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Co-authored-by: Ella Charlaix <80481427+echarlaix@users.noreply.github.com>
