[Fix] Support setting pad_val in batch collating

[AronLin]

Now, the padding value is set to 0 by default, but in gt_semantic_seg it needs to be 255.

So I modified the DefaultFormatBundle and added padding_value in the DataContainer.
In batch collating, the DataContainer will fill the data with the padding_value.

[AronLin]

I will run an HTC model and a Panoptic FPN model to ensure performance stability.

[codecov]

Codecov Report

Merging #6837 (348ce0e) into dev-v2.20.0 (5e246d5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           dev-v2.20.0    #6837      +/-   ##
===============================================
+ Coverage        62.02%   62.03%   +0.01%     
===============================================
  Files              322      323       +1     
  Lines            25717    25745      +28     
  Branches          4356     4363       +7     
===============================================
+ Hits             15951    15971      +20     
- Misses            8918     8928      +10     
+ Partials           848      846       -2     

Flag	Coverage Δ
unittests	62.01% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet/datasets/pipelines/formating.py	67.76% <100.00%> (+0.26%)	⬆️
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	68.06% <0.00%> (-1.41%)	⬇️
mmdet/apis/train.py	15.66% <0.00%> (-1.01%)	⬇️
mmdet/datasets/lvis.py	14.86% <0.00%> (ø)
mmdet/utils/__init__.py	100.00% <0.00%> (ø)
mmdet/models/backbones/pvt.py	78.15% <0.00%> (ø)
mmdet/models/detectors/lad.py	34.28% <0.00%> (ø)
mmdet/models/backbones/swin.py	81.50% <0.00%> (ø)
mmdet/models/detectors/solo.py	83.33% <0.00%> (ø)
... and 8 more

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[AronLin]

PanopticFPN: 40.1PQ.
HTC: bbox mAP:42.2, segm mAP:37.4.
They are normal.