[Feature] Remove the Nd*TensorSpec classes

[riiswa]

Description

See #771

Motivation and Context

This will close #771

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• 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 #772 (0112fb7) into main (578938a) will decrease coverage by 0.02%.
The diff coverage is 97.82%.

@@            Coverage Diff             @@
##             main     #772      +/-   ##
==========================================
- Coverage   88.73%   88.71%   -0.03%     
==========================================
  Files         123      123              
  Lines       20986    20944      -42     
==========================================
- Hits        18622    18580      -42     
  Misses       2364     2364              

Flag	Coverage Δ
habitat-gpu	24.85% <16.00%> (-0.13%)	⬇️
linux-brax	29.49% <38.96%> (-0.10%)	⬇️
linux-cpu	85.25% <93.95%> (-0.02%)	⬇️
linux-gpu	86.16% <93.40%> (-0.03%)	⬇️
linux-jumanji	30.27% <61.03%> (-0.09%)	⬇️
linux-outdeps-gpu	71.87% <77.34%> (-0.05%)	⬇️
linux-stable-cpu	85.11% <93.40%> (-0.02%)	⬇️
linux-stable-gpu	85.80% <93.40%> (-0.02%)	⬇️
linux_examples-gpu	42.91% <54.92%> (-0.04%)	⬇️
macos-cpu	84.93% <88.46%> (-0.02%)	⬇️
olddeps-gpu	75.87% <79.55%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
torchrl/collectors/collectors.py	70.84% <ø> (ø)
torchrl/data/__init__.py	100.00% <ø> (ø)
torchrl/envs/model_based/common.py	91.42% <ø> (ø)
torchrl/envs/model_based/dreamer.py	100.00% <ø> (ø)
torchrl/modules/planners/cem.py	97.61% <ø> (ø)
torchrl/modules/tensordict_module/actors.py	89.18% <ø> (ø)
torchrl/modules/tensordict_module/common.py	82.78% <ø> (ø)
torchrl/modules/tensordict_module/exploration.py	77.46% <ø> (ø)
torchrl/modules/tensordict_module/sequence.py	100.00% <ø> (ø)
torchrl/trainers/helpers/models.py	93.38% <ø> (ø)
... and 20 more

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[vmoens]

Great work, thanks so much