move python computation logic into c++

[wangsiping97]

Description

Moved python computation in _to_dm() function in dm_envpool.py to c++ implementation.

Motivation and Context

After fix, envpool's speedup increased from ~0.7x to ~0.9x:

Namespace(domain='cheetah', seed=0, task='run', total_step=200000)
100%|███████████████████████████████████████████████████████████████████████████████████████████████| 200000/200000 [00:16<00:00, 11947.27it/s]
FPS(dmc) = 11945.41
100%|███████████████████████████████████████████████████████████████████████████████████████████████| 200000/200000 [00:17<00:00, 11233.41it/s]
FPS(envpool) = 11233.09
EnvPool Speedup: 0.94x

Implemented Tasks

Added two fields in State:

• discount: default by (1.0 - done)
• step_type: aligned with dm_env.StepType value

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• New environment (non-breaking change which adds 3rd-party environment)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of example)

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.
• I have reformatted the code using make format (required)
• I have checked the code using make lint (required)
• I have ensured make bazel-test pass. (required)