Add support for exporting custom models

[xenova]

Needed for https://huggingface.co/nomic-ai/nomic-embed-text-v1

Example:

$ python -m scripts.convert --model_id nomic-ai/nomic-embed-text-v1 --opset 12 --custom_onnx_configs '{"model":"bert"}' --trust_remote_code --task feature-extraction
Framework not specified. Using pt to export to ONNX.
<All keys matched successfully>
Using framework PyTorch: 2.1.2+cu121
Overriding 1 configuration item(s)
        - use_cache -> False
/home/codespace/.cache/huggingface/modules/transformers_modules/nomic-ai/nomic-embed-text-v1/98b6aaaf6dc0f3d1fd63f6cb72593fc3b9a8ac9e/modeling_hf_nomic_bert.py:631: TracerWarning: Converting a tensor to a Python boolean might cause the trace to be incorrect. We can't record the data flow of Python values, so this value will be treated as a constant in the future. This means that the trace might not generalize to other inputs!
  if seqlen > self._seq_len_cached:
/home/codespace/.cache/huggingface/modules/transformers_modules/nomic-ai/nomic-embed-text-v1/98b6aaaf6dc0f3d1fd63f6cb72593fc3b9a8ac9e/modeling_hf_nomic_bert.py:584: TracerWarning: Converting a tensor to a Python boolean might cause the trace to be incorrect. We can't record the data flow of Python values, so this value will be treated as a constant in the future. This means that the trace might not generalize to other inputs!
  seqlen > self._seq_len_cached
/home/codespace/.cache/huggingface/modules/transformers_modules/nomic-ai/nomic-embed-text-v1/98b6aaaf6dc0f3d1fd63f6cb72593fc3b9a8ac9e/modeling_hf_nomic_bert.py:514: TracerWarning: Converting a tensor to a Python boolean might cause the trace to be incorrect. We can't record the data flow of Python values, so this value will be treated as a constant in the future. This means that the trace might not generalize to other inputs!
  assert ro_dim <= x.shape[-1]
Post-processing the exported models...
Deduplicating shared (tied) weights...
Validating ONNX model models/nomic-ai/nomic-embed-text-v1/model.onnx...
        -[✓] ONNX model output names match reference model (last_hidden_state)
        - Validating ONNX Model output "last_hidden_state":
                -[✓] (2, 16, 768) matches (2, 16, 768)
                -[✓] all values close (atol: 0.0001)
The ONNX export succeeded and the exported model was saved at: models/nomic-ai/nomic-embed-text-v1

[HuggingFaceDocBuilderDev]

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