Try to import torch_geometric only when init Raindrop

[WenjieDu]

What does this PR do?

1. fixing Only reminder missing torch_geometric when initializing Raindrop #380;

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 8959242419

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.

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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 4 of 8 (50.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.009%) to 81.666%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/nn/modules/raindrop/backbone.py	4	6	66.67%
pypots/nn/modules/raindrop/layers.py	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/nn/modules/fedformer/layers.py	2	44.2%

Totals
Change from base Build 8892490423:	0.009%
Covered Lines:	7813
Relevant Lines:	9567

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💛 - Coveralls