[Fix] Fix mix training on Ascend NPU

[xuuyangg]

Motivation

Address an issue where mix training is not enabled when optimizer_config is present in the config on Ascend NPU

Modification

Previously, mix training was not enabled when optimizer_config was present in the configuration on Ascend NPU. This commit addresses the issue by ensuring that mix training is enabled under these circumstances.

Use cases

It has been validated on the knet_s3_upernet_swin-l_8x2_640x640_adamw_80k_ade20k.py config.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[Ginray]

/lgtm

[OpenMMLab-Assistant-004]

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