[Feature] Port test_fake_tensordict to torchrl

[vmoens]

Description

Exposes test_fake_tensordict to the users as a sanity check for custom environments.

[codecov]

Codecov Report

Merging #731 (70f4c4d) into main (a677fb1) will increase coverage by 0.00%.
The diff coverage is 94.11%.

@@           Coverage Diff           @@
##             main     #731   +/-   ##
=======================================
  Coverage   88.71%   88.71%           
=======================================
  Files         120      120           
  Lines       20240    20241    +1     
=======================================
+ Hits        17955    17956    +1     
  Misses       2285     2285           

Flag	Coverage Δ
habitat-gpu	25.06% <11.76%> (+<0.01%)	⬆️
linux-cpu	85.66% <91.17%> (+<0.01%)	⬆️
linux-gpu	86.60% <91.17%> (+<0.01%)	⬆️
linux-jumanji	30.27% <88.23%> (+<0.01%)	⬆️
linux-outdeps-gpu	72.11% <11.76%> (+<0.01%)	⬆️
linux-stable-cpu	85.51% <91.17%> (+<0.01%)	⬆️
linux-stable-gpu	86.23% <91.17%> (+<0.01%)	⬆️
linux_examples-gpu	43.00% <10.34%> (-0.09%)	⬇️
macos-cpu	85.33% <91.17%> (+<0.01%)	⬆️
olddeps-gpu	76.10% <88.23%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
test/_utils_internal.py	89.70% <ø> (-2.05%)	⬇️
test/test_libs.py	96.19% <80.00%> (+0.01%)	⬆️
torchrl/envs/utils.py	95.77% <96.55%> (+0.53%)	⬆️

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