Allow Trainer.get_optimizer_cls_and_kwargs to be overridden

[apoorvkh]

What does this PR do?

Currently, Trainer builds an optimizer by loading the optimizer class and arguments from Trainer.get_optimizer_cls_and_kwargs in Trainer.create_optimizer:

transformers/src/transformers/trainer.py

Line 1076 in 6c1d0b0

optimizer_cls, optimizer_kwargs = Trainer.get_optimizer_cls_and_kwargs(self.args, opt_model)

However, this prevents the get_optimizer_cls_and_kwargs() function from being overridden. As a solution, I've changed it into an instance method (instead of a @staticmethod) and from Trainer.get_optimizer_cls_and_kwargs(args) to self.get_optimizer_cls_and_kwargs() in this PR. All existing functionality should remain as is, but this should now be extensible (if you subclass Trainer).

Note: I think this breaks the current tests, which expect get_optimizer_cls_and_kwargs to be a static method, e.g.

transformers/tests/trainer/test_trainer.py

Line 4182 in ad35309

actual_cls, optim_kwargs = Trainer.get_optimizer_cls_and_kwargs(training_args)

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Who can review?

@muellerzr and @SunMarc

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[amyeroberts]

Hi @apoorvkh, thanks for opening a PR!

Could you give some more details about how you'd like to use this method? I think it should be possible to override a staticmethod in a child class:

In [1]: class Foo:
   ...:     @staticmethod
   ...:     def foo(a, b):
   ...:         return a * b
   ...:
   ...:     def bar(self, c, d):
   ...:         return c ** d
   ...:
   ...:     def baz(self, e, f):
   ...:         return self.foo(e, f)
   ...:
   ...:
   ...: class Bar(Foo):
   ...:     @staticmethod
   ...:     def foo(a, b):
   ...:         return a + b
   ...:

In [2]: Foo.foo(2, 3)
Out[2]: 6

In [3]: Bar.foo(2, 3)
Out[3]: 5

In [4]: Foo().bar(2, 3)
Out[4]: 8

In [5]: Bar().bar(2, 3)
Out[5]: 8

In [6]: Foo().baz(2, 3)
Out[6]: 6

In [7]: Bar().baz(2, 3)
Out[7]: 5

but I might be missing what you're trying to do

[apoorvkh]

Yes, can definitely elaborate:

Say I want to use HF Trainer with an arbitrary PyTorch optimizer (AdamW here just as an example). Then I should intuitively extend Trainer like:

class CustomOptimizerTrainer(Trainer):
    @staticmethod
    def get_optimizer_cls_and_kwargs(args: HfTrainingArguments, model=None) -> tuple[type[torch.optim.Optimizer], dict[str, Any]]:
        optimizer = torch.optim.AdamW
        optimizer_kwargs = {
            "lr": 4e-3,
            "betas": (0.9, 0.999),
            "weight_decay": 0.05,
        }
        return optimizer, optimizer_kwargs

However, this won't take effect, because Trainer.create_optimizer hardcodes the Trainer class name when calling get_optimizer_cls_and_kwargs:

transformers/src/transformers/trainer.py

Line 1076 in 6c1d0b0

optimizer_cls, optimizer_kwargs = Trainer.get_optimizer_cls_and_kwargs(self.args, opt_model)

This is not self.get_optimizer_cls_and_kwargs and so CustomOptimizerTrainer.get_optimizer_cls_and_kwargs will never be called. I think the best fix is to change Trainer.get_optimizer_cls_and_kwargs to self.get_optimizer_cls_and_kwargs in the original source of Trainer.create_optimizer.

I also made get_optimizer_cls_and_kwargs an instance method instead of a static method, but that probably doesn't matter as much and can be reverted. It breaks the syntax of the tests.

Please let me know if that's clearer and if you agree! Thanks!

[amyeroberts]

@apoorvkh Thanks for taking the time for the detailed explanation! Yes, I think switching to optimizer_cls, optimizer_kwargs = self.get_optimizer_cls_and_kwargs(self.args, opt_model) should be OK, and a simple solution.

I'd rather we didn't change the method to be an instance method - this is a breaking change which might affect many users downstream.

[apoorvkh]

Okay, sounds good then! That makes this a very simple PR. Made those changes and all tests pass :)

[amyeroberts]

Thanks for this change and making our objects more flexible!

All LGTM - let's just get a seoncd 👍 from @muellerzr or @SunMarc to confirm this is all OK in trainer-land

[apoorvkh]

Thanks! I am also considering making another (simple, no breaking changes) PR to support generic PyTorch optimizers via TrainingArguments. Would you support that idea?

[muellerzr]

Makes sense to me as well, adds some nice flexibility :)

[apoorvkh]

PR to support generic PyTorch optimizers via TrainingArguments

I was going to add support for something like

TrainingArguments(
    optim=torch.optim.AdamW,
    optim_args={
        "betas" : (0.9, 0.999),
        "eps" : 1e-08,
        "weight_decay" : 0.01
    }
)

(in addition to the existing functionality of optim and optim_args for optimizers implemented in Transformers)

But looks like TrainingArguments objects must be JSON serializable. I don't have another approach in mind that is as elegant. We could allow optim="AdamW" and then do optimizer = getattr(torch.optim, args.optim). But this is specific to the optimizers available in torch.optim and the optim argument is already crowded with strings corresponding to OptimizerNames. Unless someone has a better idea, I will leave this for now :(

[tomaarsen]

Big thanks for fixing this! This should save me a massive headache in Sentence Transformers.

• Tom Aarsen