[fix] val loader should not drop last by default.

[Fei-Wang]

Motivation

Val loader should not drop last by default.

Modification

add 'drop_last': False in val_loader_cfg by default.

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

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

[Fei-Wang]

@mzr1996

[mzr1996]

The default behavior of PyTorch should be drop_last=False, is this line necessary?

[Fei-Wang]

If not explicitly specified as False, it will be set to True in some cases.

https://github.com/open-mmlab/mmclassification/blob/c94e9b36699094ca8cfd975442474ca4ac47f083/configs/vision_transformer/vit-base-p16_ft-4xb544-ipu_in1k.py#L54

https://github.com/open-mmlab/mmclassification/blob/c94e9b36699094ca8cfd975442474ca4ac47f083/mmcls/apis/train.py#L209

[mzr1996]

If not explicitly specified as False, it will be set to True in some cases.

https://github.com/open-mmlab/mmclassification/blob/c94e9b36699094ca8cfd975442474ca4ac47f083/configs/vision_transformer/vit-base-p16_ft-4xb544-ipu_in1k.py#L54

https://github.com/open-mmlab/mmclassification/blob/c94e9b36699094ca8cfd975442474ca4ac47f083/mmcls/apis/train.py#L209

Get it

[tonysy]

Thanks for your contribution. We will improve our respone SOP for community contribution. Feel free to share your ideas and comments on MMClassification.