[Fix] Fix gradient cumulative optimizer when resuming

[shinya7y]

Motivation

Fix #2083

Modification

• Fix the issue
• Eliminate the calculation of residual_iters to avoid complicated implementation of mmcv/runner/hooks/optimizer.py and unit tests (I think the logic has another bug because every_n_iters has not been modified for it.)
• Put the same code into _get_loss_factor

BC-breaking (Optional)

Since the bug causes wrong loss, models trained by simultaneous use of GradientCumulativeOptimizerHook and resume will show different results before and after this PR.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[HAOCHENYE]

Thanks for your contribution! LGTM~