[RLlib] DM control suite wrapper fix: dtype of obs needs to be pinned to float32.

[sven1977]

DM control suite wrapper fix: dtype of observation returned by _get_obs() method needs to be pinned to float32.
If not, this might lead to a type mismatch (float32 matrix vs float64 observation) in a network that processes these observations, especially after an env reset.

Why are these changes needed?

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