[BugFix] Reinstantiate custom value key for multioutput value networks

[vmoens]

Description

Reset the default value_key in the advantages modules as the first key may not be the value.
An additional check is run to make sure the key can be found in the outputs from the module

cc @albertbou92

[codecov]

Codecov Report

Merging #754 (56b0913) into main (1f8341c) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #754      +/-   ##
==========================================
- Coverage   88.77%   88.76%   -0.02%     
==========================================
  Files         123      123              
  Lines       20788    20794       +6     
==========================================
+ Hits        18454    18457       +3     
- Misses       2334     2337       +3     

Flag	Coverage Δ
habitat-gpu	25.14% <0.00%> (-0.02%)	⬇️
linux-brax	29.47% <0.00%> (-0.02%)	⬇️
linux-cpu	85.28% <66.66%> (-0.02%)	⬇️
linux-gpu	86.21% <66.66%> (+<0.01%)	⬆️
linux-jumanji	30.20% <0.00%> (-0.02%)	⬇️
linux-outdeps-gpu	72.05% <66.66%> (-0.01%)	⬇️
linux-stable-cpu	85.13% <66.66%> (-0.02%)	⬇️
linux-stable-gpu	85.84% <66.66%> (-0.03%)	⬇️
linux_examples-gpu	42.99% <0.00%> (-0.03%)	⬇️
macos-cpu	84.96% <66.66%> (-0.02%)	⬇️
olddeps-gpu	75.92% <66.66%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
torchrl/objectives/value/advantages.py	88.33% <66.66%> (-1.33%)	⬇️

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