[Feature] FrameSkipTransform

[vmoens]

Description

Introduces the FrameSkipTransform
Refactors the transformed environment parent by adding a caching mechanism.
Tests that the cache is emptied whenever the transform list is changed.

cc @albertbou92 any other idea of changes in the transformed env that might break the caching of the parent (have a look at the test).

Maybe we'd like to allow

env = TransformedEnv(base_env, transform1)
env.transform = transform2
assert transform1.parent.base_env is not env # should not be env because we have done a substitution

which I think now would break

EDIT

I now made this possible

[codecov]

Codecov Report

Merging #749 (a8d0f34) into main (427c89d) will increase coverage by 12.79%.
The diff coverage is 95.45%.

❗ Current head a8d0f34 differs from pull request most recent head 82bee08. Consider uploading reports for the commit 82bee08 to get more accurate results

@@             Coverage Diff             @@
##             main     #749       +/-   ##
===========================================
+ Coverage   76.01%   88.80%   +12.79%     
===========================================
  Files         123      123               
  Lines       20904    20886       -18     
===========================================
+ Hits        15890    18548     +2658     
+ Misses       5014     2338     -2676     

Flag	Coverage Δ
habitat-gpu	25.13% <11.39%> (+0.16%)	⬆️
linux-brax	29.46% <11.39%> (-0.13%)	⬇️
linux-cpu	85.36% <95.45%> (?)
linux-gpu	86.27% <95.45%> (?)
linux-jumanji	30.18% <11.39%> (-0.18%)	⬇️
linux-outdeps-gpu	71.92% <57.79%> (-0.10%)	⬇️
linux-stable-cpu	85.21% <95.45%> (?)
linux-stable-gpu	85.91% <95.45%> (?)
linux_examples-gpu	43.00% <62.02%> (+0.05%)	⬆️
macos-cpu	85.04% <95.45%> (?)
olddeps-gpu	76.02% <92.85%> (?)

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

Impacted Files	Coverage Δ
torchrl/envs/transforms/__init__.py	100.00% <ø> (ø)
torchrl/envs/transforms/functional.py	75.00% <ø> (ø)
torchrl/envs/transforms/transforms.py	87.35% <90.41%> (+3.91%)	⬆️
test/test_transforms.py	96.56% <100.00%> (+26.70%)	⬆️
torchrl/envs/common.py	83.93% <100.00%> (+7.86%)	⬆️
torchrl/envs/libs/dm_control.py	84.83% <100.00%> (+10.35%)	⬆️
torchrl/collectors/utils.py	76.31% <0.00%> (-0.61%)	⬇️
torchrl/envs/libs/jumanji.py	90.47% <0.00%> (-0.28%)	⬇️
test/test_postprocs.py	97.56% <0.00%> (ø)
torchrl/envs/libs/brax.py	92.15% <0.00%> (ø)
... and 47 more

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