Renaming d_inner into d_ffn

[WenjieDu]

What does this PR do?

1. fixing Unify the name of argument d_inner and d_ffn #324;

Before submitting

• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 8471955669

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 4 of 5 (80.0%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 82.083%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/nn/modules/transformer/layers.py	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/usgan/model.py	1	88.0%

Totals
Change from base Build 8471860827:	-0.02%
Covered Lines:	5626
Relevant Lines:	6854

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💛 - Coveralls