Add the joint pos obs back

[squarefk]

What this does

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Title	Label
Fixes #[issue]	(🐛 Bug)
Optimizes something	(⚡️ Performance)
Adds a new feature	(✨ Feature)

How it was tested

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• Added test_something in tests/test_stuff.py.
• Added new_feature and checked that training converges with policy X on dataset/environment Y.
• Optimized some_function, it now runs X times faster than previously.

How to checkout & try? (for the reviewer)

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DATA_DIR=tests/data pytest -sx tests/test_stuff.py::test_something

python robosuite/scripts/train.py --some.option=true

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