refactor TrainPipelineBase to clean input batch after the forward pass

[TroyGarden]

Summary:

context

• previously in the TrainPipelineBase, the cur_batch (model input) is not released until calling the loss.backward().
• however, the cur_batch is only needed during the forward pass.
• this diff changes the order of clearing the current batch so that it's cleared right after the forward pass

NOTE: usually the peak memory usage happens at the beginning of the backward pass, so clearing the unused input batch can reduce the peak memory usage.

• benchmark comparison indicates roughly 1~1.5x of memory saving (input batch ~ 1GB)

name	GPU Peak Memory alloc	GPU Peak Memory reserved
before	35.94 GB	56.72 GB
after	34.33 GB	54.00 GB
before-inplace	35.94 GB	53.91 GB
after-inplace	34.33 GB	51.35 GB

NOTE: in-place copy batch to gpu won't change the gpu peak memory allocation, but can reduce the peak memory reservation.

Differential Revision: D85483966

[meta-codesync]

@TroyGarden has exported this pull request. If you are a Meta employee, you can view the originating Diff in D85483966.