Fix v2 model loading issue
Author: BigBird01
Pull request: yes

# What does this PR do?
Fix few issues with loading deberta v2 models and deberta mnli fine-tuned model
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Comments:
Hi @BigBird01, thanks a lot for fixing these issues! I'd like to prevent as much as possible from having the `pre_load_hooks` in the code. When do you expect mismatched head dimensions?

This is the case that if we want to fine-tune a task based on mnli models, e.g. MRPC, SST, QNLI. If we want to avoid this method, we need to fix the error reporting when load pretrained models.

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From: Lysandre Debut <notifications@github.com>
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To: huggingface/transformers <transformers@noreply.github.com>
Cc: Pengcheng He <biglm100@gmail.com>; Mention <mention@noreply.github.com>
Subject: Re: [huggingface/transformers] Fix v2 model loading issue (#10129)


Hi @BigBird01<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FBigBird01&data=04%7C01%7CPengcheng.H%40microsoft.com%7C301190f89ac34907b53408d8d0229db7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637488194400010761%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=9%2BnuKLc3nPym5xSUhe%2Flko3Fd3jxus0WRwj3W%2Bks%2FuM%3D&reserved=0>, thanks a lot for fixing these issues! I'd like to prevent as much as possible from having the pre_load_hooks in the code. When do you expect mismatched head dimensions?

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Usually we recommend to load through the base model to lose the head:

```py
from transformers import DebertaV2Model, DebertaV2ForSequenceClassification

seq_model = DebertaV2ForSequenceClassification.from_pretrained("xxx", num_labels=4)
seq_model.save_pretrained(directory)

base = DebertaV2Model.from_pretrained(directory)  # Lose the head
base.save_pretrained(directory)

seq_model = DebertaV2ForSequenceClassification.from_pretrained(directory, num_labels=8)
```

Does that work in your case? I agree you're touching to something that has a bad API, and this should be handled in the `from_pretrained` method. I don't think we should handle it model-wise, however. I'll look into it soon.

yes. but this looks a little bit tricky. And need to modify existing text classification code to benefit from mnli fine-tuned models. How about we keep current hook method, and finish current PR? After we work out a systematic solution for such scenario, we can drop the hook method.

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From: Lysandre Debut <notifications@github.com>
Sent: Saturday, February 13, 2021 6:28:34 AM
To: huggingface/transformers <transformers@noreply.github.com>
Cc: Pengcheng He <biglm100@gmail.com>; Mention <mention@noreply.github.com>
Subject: Re: [huggingface/transformers] Fix v2 model loading issue (#10129)


Usually we recommend to load through the base model to lose the head:

from transformers import DebertaV2Model, DebertaV2ForSequenceClassification

seq_model = DebertaV2ForSequenceClassification.from_pretrained("xxx", num_labels=4)
seq_model.save_pretrained(directory)

base = DebertaV2Model.from_pretrained(directory)  # Lose the head
base.save_pretrained(directory)

seq_model = DebertaV2ForSequenceClassification.from_pretrained(directory, num_labels=8)

Does that work in your case? I agree you're touching to something that has a bad API, and this should be handled in the from_pretrained method. I don't think we should handle it model-wise, however. I'll look into it soon.

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Ok, will merge like that and we'll discuss with other team members for the main PR. Thanks!