Install docs fix: MMCV >= 1.3.8, < 1.4, but not 1.3.3

[drscotthawley]

Fixes MMCV==1.3.3 is used but incompatible. Please install mmcv>=1.3.8, <=1.4.0.

BTW, here's some code I cooked up that may be of interest to other users. Didn't put in in PR itself but would be happy to do so:

import torch, re 
tv, cv = torch.__version__, torch.version.cuda
tv = re.sub('\+cu.*','',tv)
TORCH_VERSION = 'torch'+tv[0:-1]+'0'
CUDA_VERSION = 'cu'+cv.replace('.','')

print(f"TORCH_VERSION={TORCH_VERSION}; CUDA_VERSION={CUDA_VERSION}")

!pip install -qq mmcv-full=="1.3.8" -f https://download.openmmlab.com/mmcv/dist/{CUDA_VERSION}/{TORCH_VERSION}/index.html --upgrade
!pip install mmdet -qq

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.64%. Comparing base (8716362) to head (236d167).
Report is 97 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #925      +/-   ##
==========================================
+ Coverage   86.57%   86.64%   +0.06%     
==========================================
  Files         250      250              
  Lines        5384     5412      +28     
==========================================
+ Hits         4661     4689      +28     
  Misses        723      723              

Flag	Coverage Δ
unittests	86.64% <100.00%> (+0.06%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ai-fast-track]

Thanks a lot Scott for your contributions.

MMLabs recently added a new package that can automatically handle both mmcv and mmdet installation by detecting which torch, torhvision, and cuda versions are installed.

I submitted a PR to include that but we are facing unrelated issues (related to a zip file: sigh).

Here is the recommended way of mmdet/mmcv installation:

!pip install openmim -q
!echo "- Installing mmcv"
!mim install mmcv-full
!echo "- Installing mmdet"
!mim install mmdet

[FraPochetti]

@ai-fast-track thoughts on this one?