further speedup

[wangsiping97]

Description

Cache spec attr to speedup, profile result says reading self._spec.xxx takes a lot of time.

Motivation and Context

After fix, envpool's speedup increased from ~0.9x to ~1.1x:

Namespace(domain='cheetah', seed=0, task='run', total_step=200000)
100%|███████████████████████████████████████████████████████████████████████████████████████████████| 200000/200000 [00:16<00:00, 12291.98it/s]
FPS(dmc) = 12289.97
100%|███████████████████████████████████████████████████████████████████████████████████████████████| 200000/200000 [00:14<00:00, 14146.43it/s]
FPS(envpool) = 14145.82
EnvPool Speedup: 1.15x

• 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? 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)

Implemented Tasks

• Subtask 1
• Subtask 2
• Subtask 3

Checklist

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• 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)