[Doc] Refactor DDPG and DQN tutos to narrow the scope

[vmoens]

Description

Narrows the scope of DDPG and DQN tutos:

• DDPG now gives the instruction to write a loss module.
• DQN gives the instruction to use TorchRL's trainer class.

[codecov]

Codecov Report

Merging #979 (86aefc2) into main (ae53793) will decrease coverage by 0.04%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main     #979      +/-   ##
==========================================
- Coverage   90.07%   90.03%   -0.04%     
==========================================
  Files         148      148              
  Lines       31476    31521      +45     
==========================================
+ Hits        28351    28380      +29     
- Misses       3125     3141      +16     

Flag	Coverage Δ
d4rl-gpu	29.39% <34.09%> (-0.07%)	⬇️
habitat-gpu	27.59% <11.36%> (-0.05%)	⬇️
linux-brax	30.46% <11.36%> (-0.06%)	⬇️
linux-cpu	85.42% <78.02%> (-0.03%)	⬇️
linux-distributed-gpu	35.88% <11.36%> (-0.36%)	⬇️
linux-envpool	23.81% <11.36%> (-0.04%)	⬇️
linux-gpu	83.18% <78.02%> (-0.02%)	⬇️
linux-jumanji	28.39% <11.36%> (-0.05%)	⬇️
linux-outdeps-gpu	74.88% <61.53%> (-0.05%)	⬇️
linux-stable-cpu	85.32% <78.02%> (-0.03%)	⬇️
linux-stable-gpu	86.16% <78.02%> (-0.06%)	⬇️
linux-vmas	30.59% <11.36%> (-0.06%)	⬇️
linux_examples-gpu	62.88% <66.29%> (-0.01%)	⬇️
macos-cpu	85.16% <78.02%> (-0.03%)	⬇️
olddeps-gpu	76.36% <61.53%> (-0.05%)	⬇️
sklearn-gpu	28.04% <19.31%> (-0.09%)	⬇️

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

Impacted Files	Coverage Δ
torchrl/modules/__init__.py	100.00% <ø> (ø)
torchrl/modules/tensordict_module/__init__.py	100.00% <ø> (ø)
torchrl/objectives/ddpg.py	98.59% <ø> (+1.33%)	⬆️
torchrl/objectives/dqn.py	93.97% <ø> (ø)
torchrl/objectives/sac.py	92.09% <ø> (ø)
torchrl/objectives/value/advantages.py	82.72% <ø> (ø)
torchrl/envs/transforms/transforms.py	87.31% <56.00%> (-0.52%)	⬇️
torchrl/trainers/trainers.py	87.59% <92.85%> (-0.72%)	⬇️
test/test_trainer.py	98.07% <100.00%> (-0.17%)	⬇️
torchrl/data/__init__.py	100.00% <100.00%> (ø)
... and 9 more

... and 1 file with indirect coverage changes

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