Refactor the baseclass related to transformer

[jshilong]

PR Message

This pr refactor the FFN and BaseTransformerLayer.

1. Add registry named DROPOUT_LAYERS and support to specify the DropPath or Dropout when adding the residue in the Attention or FFN.
2. Add registry named FEEDFORWARD_NETWORK.
3. Support to specify different probability for MultiheadAttention . with attn_drop and proj_drop
4. Move all FFN related arguments to a list of Dict named ffn_cfgs so that we can build the FFN more flexibly.
5. Change all nn.ModuleList to Moduleist.
6. Support batch_first arguments for MultiheadAttention.
7. Move MultiScaleDeformableAttention to mmcv/ops/multi_scale_deform_attn.py so that mmcv can be used without compiling ops.

BC-breaking

from mmcv.cnn.bricks.transformer import MultiScaleDeformableAttention should be changed to from mmcv.ops.multi_scale_deform_attn import MultiScaleDeformableAttention

[codecov]

Codecov Report

Merging #978 (65bcd3f) into master (4d42365) will increase coverage by 2.29%.
The diff coverage is 74.86%.

@@            Coverage Diff             @@
##           master     #978      +/-   ##
==========================================
+ Coverage   65.20%   67.50%   +2.29%     
==========================================
  Files         156      159       +3     
  Lines       10034    10186     +152     
  Branches     1816     1847      +31     
==========================================
+ Hits         6543     6876     +333     
+ Misses       3154     2948     -206     
- Partials      337      362      +25     

Flag	Coverage Δ
unittests	67.50% <74.86%> (+2.29%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/ops/multi_scale_deform_attn.py	58.40% <53.75%> (-11.29%)	⬇️
mmcv/cnn/bricks/transformer.py	81.62% <86.48%> (+81.62%)	⬆️
mmcv/cnn/bricks/__init__.py	100.00% <100.00%> (ø)
mmcv/cnn/bricks/drop.py	100.00% <100.00%> (ø)
mmcv/cnn/bricks/registry.py	100.00% <100.00%> (ø)
mmcv/ops/__init__.py	100.00% <100.00%> (ø)
mmcv/onnx/info.py	68.75% <0.00%> (-6.25%)	⬇️
mmcv/tensorrt/tensorrt_utils.py	0.00% <0.00%> (-2.39%)	⬇️
mmcv/fileio/parse.py	100.00% <0.00%> (ø)
mmcv/runner/hooks/evaluation.py	88.15% <0.00%> (ø)
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4d42365...65bcd3f. Read the comment docs.

[CLAassistant]

All committers have signed the CLA.

[nbei]

Just suggestions.