[Feature] Add PoseWarper detector model

[liqikai9]

Motivation

To support input multiple frames as a single sample when dealing with video dataset, we add a new detector PoseWarper which is inherited from TopDown. The main difference between PoseWarper and TopDown lies in that the former takes a list of tensors as input while the latter takes a single tensor as input.

To support PoseWarper, we add a new neck PoseWarperNeck in mmpose/models/necks/posewarper_neck.py.

Modification

1. Add mmpose/models/detectors/posewarper.py.
   Modify mmpose/models/detectors/__init__.py

2. Add mmpose/models/necks/posewarper_neck.py.
   Modify mmpose/models/necks/__init__.py.

3. To support freezing certain stages in the backbone HRNet, also modify mmpose/models/backbones/hrnet.py.
   Modify mmpose/models/detectors/top_down.py, fix typo.

4. Add related unittests in tests/test_backbones/test_hrnet.py, tests/test_models/test_top_down_forward.py and tests/test_necks/test_posewarper_neck.py.

5. To support more flexible input format, modify tools/analysis/get_flops.py.

[luminxu]

1. According to our previous discussion, maybe we first merge the individual modules (dataset, detector, etc) into a new branch (e.g. posewarper). After the whole algorithm is finished and verified, we merge it to the master.
2. Maybe we should rename the detector as posewarper.py or something else, as this detector does not work for all the top-down video pose estimation methods. For example, some top-down video methods merge features in another way, or apply different backbones for different frames.

[codecov]

Codecov Report

Merging #932 (170f5a6) into dev-0.21 (c0d5e4c) will increase coverage by 0.79%.
The diff coverage is 84.48%.

@@             Coverage Diff              @@
##           dev-0.21     #932      +/-   ##
============================================
+ Coverage     82.59%   83.39%   +0.79%     
============================================
  Files           191      196       +5     
  Lines         14407    15142     +735     
  Branches       2491     2736     +245     
============================================
+ Hits          11899    12627     +728     
+ Misses         1863     1823      -40     
- Partials        645      692      +47     

Flag	Coverage Δ
unittests	83.32% <84.48%> (+0.79%)	⬆️

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Impacted Files	Coverage Δ
mmpose/apis/inference.py	48.63% <0.00%> (ø)
mmpose/datasets/__init__.py	100.00% <ø> (ø)
mmpose/datasets/datasets/__init__.py	100.00% <ø> (ø)
...s/datasets/top_down/topdown_posetrack18_dataset.py	93.38% <ø> (+66.17%)	⬆️
mmpose/models/backbones/vipnas_resnet.py	93.95% <ø> (+3.77%)	⬆️
mmpose/apis/train.py	20.89% <26.66%> (+0.14%)	⬆️
mmpose/datasets/pipelines/top_down_transform.py	65.14% <28.57%> (-1.72%)	⬇️
mmpose/datasets/pipelines/shared_transform.py	85.85% <33.33%> (-1.22%)	⬇️
mmpose/datasets/pipelines/loading.py	82.60% <66.66%> (-4.06%)	⬇️
mmpose/models/detectors/top_down.py	72.03% <75.00%> (+4.23%)	⬆️
... and 20 more

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