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* [feat] FSDP: supporting multiple flatten parameter groups

- step 3: make FSDP use FlattenParamModule unconditionally

* fixing the auto_wrap tests

* minor

* rewrite local_metadata_dict

- updated FPW so that custom flat param name is also supported

* bug fix

* mypy

* rewrote consolidate_shard_weights

- test_consolidate passes

* comments

* fixing pickling

* Fix shared params and MoE logic (#749)

* add strict kwarg to support fairseq:gshard MoE saving logic

* Test fairseq style shard

* style

* formatting and address comments

* added changelog

* fixing a test after padding renaming
Co-authored-by: Min Xu <min.xu.public@gmail.com>
Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

* refactoring FlattenParamWrapper

- use a FlatParameter class to encapsulate the logic of
  flattening and expanding into views.
- this will make it easier to have multiple groups of flatten
  parameters

* fixed testing context issues for both temp files and temp dirs

* fixing test_fsdp_metadata

* fix pickling of FlatParameter

* fixed test_fsdp_optimizer_utils.py

* minor

* fix assert

* lint

* remove nesting from the test

* step 1.5: remove the code related unnecessary nesting support in FPW

* Update fairscale/nn/misc/flatten_params_wrapper.py
Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

* address comment
Co-authored-by: Min Xu <min.xu.public@gmail.com>
Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

* Save FSDP metadata for offline unflattening

* Complete the meta-data saving method with all the information needed to reconstruct a checkpoint offline, and implement the method that reconstruct a consolidated checkpoint from a sharded checkpoint

* Complete the meta-data saving method with all the information needed to reconstruct a checkpoint offline, and implement the method that reconstruct a consolidated checkpoint from a sharded checkpoint

* Add a unit test to show how to use the function

* Code review + improvement of the unit tests

* Code review: extract clean_path

* Make meta data and consolidation of checkpoint work for flatten_parameter=False

* Add new unit test file in CI

* Complete changelog and fix mypy issues

* Add support for module buffers in the consolidation of sharded checkpoints

* Better support for module buffers: save them in the meta data

* Refactoring: use a data-format for the meta data that is simpler to understand (move from object of array to array of object format)

* Renaming to make code clearer

* Code review: in_temporary_directory rework and typo correction

* Renaming
Co-authored-by: Sam Shleifer <sshleifer@gmail.com>
Co-authored-by: QuentinDuval <QuentinDuval@users.noreply.github.com>