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Before this commit, output tensors of checkpointed modules always
require grad, even if they shouldn't. This commit makes it so that
the outputs of checkpointed modules only require grad if either
the input requires grad or if the parameters require grad.

To achieve this, this commit also adds a new _unflattened_param_views
attribute to modules being flattened. This allows the checkpointing
to still access the parameters and check if gradients need to be
computed.
Co-authored-by: Alex Xiao <axiao@fb.com>

* [test] Added disable_checkpointing unit test

* [test] Added disable_checkpointing unit test (Clean-up)

* [test] Added disable_checkpointing unit test (Clean-up)

* [do not merge] testing a corner case

* workaround

* using dummy tensor to fix

* lint

* changelog

* update a comment
Co-authored-by: Min Xu <min.xu.public@gmail.com>

* rename files

* add newly renamed file

* rename and move checkpoint activations related files

* add test files to ci list

* fix lint errors

* modify docs

* add changelog

* retain old path for now

* fix lint errors

* add another import test case

* fix merge conflict

* add missing test file