Add timm backbones to mmdet models

[ai-fast-track]

I added mmdet + timm support. The API is very similar to the existing one. I separate the namespaces in order to be able to navigate through the different backbones families and have the autocompletion kicking in.

Both mobilenet and resne(s)t backbones plugged to a retinanet model are provided as an example showing how a user could add support for a specific backbone/model combination. Given we have over 250 timm backbones and over 50 mmet models, it isn’t realistic to imagine a support for all backbone/model combinations. In both the documentation and the notebook, we show all the different ways to plug a Timm backbone to an MMDetection model.

closes #847

[codecov]

Codecov Report

Merging #848 (579c3fe) into master (944b47c) will decrease coverage by 0.55%.
The diff coverage is 80.61%.

@@            Coverage Diff             @@
##           master     #848      +/-   ##
==========================================
- Coverage   87.62%   87.06%   -0.56%     
==========================================
  Files         232      248      +16     
  Lines        4994     5367     +373     
==========================================
+ Hits         4376     4673     +297     
- Misses        618      694      +76     

Flag	Coverage Δ
unittests	87.06% <80.61%> (-0.56%)	⬇️

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

Impacted Files	Coverage Δ
...det/models/retinanet/backbones/mmdet/resnet_fpn.py	100.00% <ø> (ø)
...mdet/models/retinanet/retinanet_no_backbone_fpn.py	0.00% <0.00%> (ø)
icevision/models/mmdet/backbones/timm/resnest.py	41.81% <41.81%> (ø)
...vision/models/mmdet/backbones/timm/efficientnet.py	76.25% <76.25%> (ø)
icevision/models/mmdet/utils.py	70.70% <80.00%> (+4.04%)	⬆️
icevision/models/mmdet/backbones/timm/mobilenet.py	87.50% <87.50%> (ø)
icevision/models/mmdet/backbones/timm/resnet.py	92.72% <92.72%> (ø)
icevision/models/mmdet/__init__.py	100.00% <100.00%> (ø)
icevision/models/mmdet/backbones/timm/__init__.py	100.00% <100.00%> (ø)
icevision/models/mmdet/backbones/timm/common.py	100.00% <100.00%> (ø)
... and 28 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 944b47c...579c3fe. Read the comment docs.

[lgvaz]

Seems good on a high level, just a couple of things to address before we merge:

• Carefully handle timm imports, right now doing from icevision.all import * will try to import timm no matter if it's installed or not
• remove commented out code
• rename backbone names that start with ice

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB