Fix error in gene_random_walk

[WenjieDu]

What does this PR do?

1. train_X_ori and val_X_ori didn't get standardized, fix in this PR;

Before submitting

• 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 8892484199

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

• 7 of 9 (77.78%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 81.628%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/data/generating.py	7	9	77.78%

Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/gpvae/model.py	1	85.42%

Totals
Change from base Build 8891085445:	-0.04%
Covered Lines:	7802
Relevant Lines:	9558

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💛 - Coveralls