[Feature] Unfold transforms for folded TransformedEnv

[alexanderlobov]

Description

TransformedEnv of a TransformedEnv should just append the transforms, not create a new TransformedEnv
close #602

Failing tests investigation is in progress

Types of changes

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• New feature (non-breaking change which adds core functionality)

Checklist

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• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #630 (de7f2b1) into main (6f9faaa) will increase coverage by 0.00%.
The diff coverage is 95.00%.

@@           Coverage Diff           @@
##             main     #630   +/-   ##
=======================================
  Coverage   87.45%   87.46%           
=======================================
  Files         124      124           
  Lines       22850    22882   +32     
=======================================
+ Hits        19984    20014   +30     
- Misses       2866     2868    +2     

Flag	Coverage Δ
linux-cpu	85.87% <95.00%> (+0.01%)	⬆️
linux-gpu	87.27% <95.00%> (+0.02%)	⬆️
linux-outdeps-gpu	76.33% <95.00%> (+0.04%)	⬆️
linux-stable-cpu	85.87% <95.00%> (+0.02%)	⬆️
linux-stable-gpu	87.27% <95.00%> (+0.02%)	⬆️
macos-cpu	85.65% <95.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
torchrl/envs/transforms/transforms.py	85.88% <92.85%> (+0.09%)	⬆️
test/test_transforms.py	95.82% <100.00%> (+0.04%)	⬆️

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[vmoens]

LGTM thanks for this