Adds `PreTrainedModel.framework` attribute
# What does this PR do?
This PR introduces an attribute called `framework` in `PreTrainedModel`, `FlaxPreTrainedModel`, and `TFPreTrainedModel`. The purpose of this attribute is to allow a user to know what framework a provided model is in, as that information is not currently very accessible.

I'm a little confused as to whether this is correctly implemented. I was basing it off of the implementation of `base_model_prefix`, which doesn't have a getattr in `FlaxPretrainedModel` and `TFPretrainedModel` despite those not (AFAICT) inheriting from `PreTrainedModel`.

## Who can review?

@patil-suraj @LysandreJik 
Great, thank you @StellaAthena! I'm not sure I see the full picture of adding that argument - but I'm definitely not opposed to it if it's helpful for your use case. It's more robust than relying on the class name.

I believe the property implemented in PyTorch (and not implemented in Flax and TensorFlow) isn't voluntary - the former was implemented early (two years ago), and the latter was overlooked.

For this property in particular (`framework`), I believe having it as a simple attribute should be enough for all three frameworks.

Thanks for offering a PR!<|||||>Thanks for the PR!

The `base_model_prefix` serves a different purpose here, it indicates the module name used for the base module in a model with a specific head on top. For example, the `base_model_prefix` for bert is `bert`, which is used by the head models as the module name for the base model 

https://github.com/huggingface/transformers/blob/8bbb53e20b7873ba7f63be70d4d798e0c3568bfa/src/transformers/models/bert/modeling_bert.py#L1486-L1492

This attribute is useful when loading a base model weights into a model with a head. And the reason `base_model` property is only added in PT `PreTrainedModel` and not in `FlaxPreTrainedModel` is because in pt it's possible to return a submodule and using this the user can access the base model if he needs (for example to freeze the base).
This is not possible for example in flax, because flax modules are stateless, and returning base_model will return a reference to the module without weights. Hope this makes it clear.

And for this property `framework`, IMO we could simply add it as a getter property and return the framework string, adding it just as a getter will also prevent users from accidentally setting it.<|||||>Thank you both for the explication, it makes understanding why the `transformers` code is the way it is.

> Great, thank you @StellaAthena! I'm not sure I see the full picture of adding that argument - but I'm definitely not opposed to it if it's helpful for your use case. It's more robust than relying on the class name.

When writing code that takes a user-defined `transformers` model as an input there are a lot of weird gotchas. The impetus for this PR was my attempt to generalize [Google's BIG Bench](https://github.com/google/BIG-bench) to work with arbitrary `transformer` models, but I suspect it'll also be useful to [EleutherAI's LM Eval Harness](https://github.com/eleutherai/lm-evaluation-harness) and other similar projects. Unfortunately, there are important properties of models that are impossible to derive from the `config` file. Another example of this is the fact that some tokenizers auto-append <eot> to the end of generations while others do not.

> And for this property `framework`, IMO we could simply add it as a getter property and return the framework string, adding it just as a getter will also prevent users from accidentally setting it.

That's an interesting idea. My thought was that this approach would cause it to be encoded in `config` files, which seems like a good best practice to follow.<|||||>@patil-suraj I have updated the code to follow your suggestion. The failing tests seem to have to do with an indentation error that I cannot work out. I even copied an existing function rather than write my own, in case there was something funky about how my keyboard was registering!

**Edit:** it looks like I was being fooled by a misleading error message! Changing `string` to `str` solved the problem.<|||||>@LysandreJik @patil-suraj I don't think I can do any more. I'm having trouble installing Jax, which may be the blocker? IDK. The below image shows me running `make fixup` and then the verification test that the readout asks me to run.

<img width="1203" alt="Screen Shot 2021-10-07 at 3 21 29 PM" src="https://user-images.githubusercontent.com/15899312/136450365-266bc51f-9456-40a3-92dd-9f4f934c8544.png"><|||||>Hi @StellaAthena no problem. I could take care of this, would it be okay if I push to your branch?<|||||>> Hi @StellaAthena no problem. I could take care of this, would it be okay if I push to your branch?

Absolutely! Thanks<|||||>Thanks for working on the PR @StellaAthena!