Add basic PEFT support to train script + record module (#1411)

* Add basic support for PEFT adapter methods

This changes adds support for training policies with much less parameters
by applying adapter methods such as LoRA on specific parts of the policies
and therefore possibly higher learning rates / batch sizes.

To make this as accessible as possible I thought it useful to provide
defaults for `target_modules` and `modules_to_save`. Currently only SmolVLA
has such defaults but when we agree that this change is useful I will set
out to generate more such defaults. While the user can override these
settings, they are expected to only change the peft_method, rank and init_type
parameters.

* Implement loading of PEFT adapters

Loading a PEFT adapter is currently done by initializing a policy with default config
and then applying the adapter on the resulting model. This has the obvious drawback
that any configurations done during training are not applied in the adapted model.

Currently the `use_peft` attribute of `PreTrainedConfig` is only set during loading
to signal the following code that it has to deal with a PEFT adapter. However
we could imagine a scenario where this is already set at training time and stored
alongside the adapter.

* Store policy config alongside PEFT checkpoint

Before this change the PEFT-wrapped policy did not save the policy's config
alongside the adapter config / weights which prevented us from changing the
policy config. Now the policy config is saved both in full training and PEFT
training.

This change makes loading the PEFT policy adapter much easier as well.

* Add default config for ACT

* Support targets like `all-linear`

* Formatting

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Fix failing tests

* Remove PEFT compatibility changes in config

We'll wait for the PEFT release that fixes this for good.

* Remove `use_peft` parameter from training script

Instead we make the PEFT config optional which has the same effect.

* Log adapter config to WandB

* Better documentation for CLI arguments

* Don't unload & merge the PEFT model

This can make things hard when using quantized layers (user expects quantized base layers with
unquantized adapters for example, merging defaults to upcast the layers leading to higher
memory).

* Correct way of identifying when to save config

* Add CLI end-to-end tests

Currently there don't seem to be any way to test the CLI commands.
Since this change mostly happens in those I thought it best to add
a way to test these commands end-to-end.

More integrated commands like `lerobot-record` need patching but
standalone commands like training seem to work fine.

* Update default targets

Removed ACT since it doesn't make sense to fine-tune ACT without having it pretrained beforehand.
SmolVLA and Pi0/0.5 are much more senseful targets.

* Clean up loading code

- Centralized instantiation of the PEFT wrapper in `make_policy` for inference
  (e.g. in `lerobot-record`)
- Training a PEFT policy also sets `cfg.use_peft` so that all inference code loading
  the policy can rely on that attribute to identify if PEFT loading is needed
- Modified RTC example to also include PEFT policies. Mostly because this is an example
  I'm currently exploring.

* Make sure push_to_hub works

Since PEFT only wraps `push_to_hub` and not `push_model_to_hub`, the reference
to `self` in `policy.push_model_to_hub` is the unwrapped policy which, of course,
doesn't know anything about PEFT.

To make the upload process aware of PEFT, we pass the unwrapped policy down to
`push_model_to_hub` as a kwarg. This is not ideal but I think it is the best way
for now.

* formatting

* Warn when encountering from-scratch-training

* Revamp pretrained model loading

There were quite a few factors that convinced me that the status quo
is able to load pretrained models from the PEFT adapter config but
in fact that didn't work.

This commit fixes the following things:
- policies wrapped in PEFT will now have a `name_or_path` attribute
  containing the name or path of the pretrained model we're fine-tuning
- we further assume that SmolVLA without `pretrained_path` and
  `load_vlm_weights==False` must be an user-side error
- we assume that using PEFT on from-scratch-policies must be
  an user-side-error

* Make it possible to unset policy features

This is necessary to train pre-trained policies on new datasets so that the
features are inferred from the new dataset and not from the pretrained
policy.

* Use correct loading for PEFT in RTC example

* Make it possible to use PeftModels in eval

* Add test checking that PEFT actually reduces params

* Adapt state/action projections instead of full-finetuning

There doesn't seem to be a benefit to fully fine-tune these layers
over just adapting them, so we do that instead.

* Disallow PEFT training on non-pretrained policies

At first I thought it would make sense to have this feature
in case you want to fine-tune a pre-trained section but in the
end it makes more trouble than it's worth.

It's still possible to allow this in the future when a concrete
need arises.

* Add basic documentation

* Formatting

* Add peft as extra dependency, mark tests

Fast tests currently fail because of the missing dependency.

* Fix pre-commit issues

* Add walx <> peft conflict for uv

* Exclude peft from pi install for now

---------

Co-authored-by: nemo <git@ningu.net>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Pepijn <138571049+pkooij@users.noreply.github.com>

docs/source/_toctree.yml

@@ -19,6 +19,8 @@
     title: Train RL in Simulation
   - local: multi_gpu_training
     title: Multi GPU training
+  - local: peft_training
+    title: Training with PEFT (e.g., LoRA)
   title: "Tutorials"
 - sections:
   - local: lerobot-dataset-v3