[docs] Cannot generate MlirEnv class documentation because it is not defined

[ChrisCummins]

🐛 Bug

The MlirEnv class is only conditionally included in the compiler_gym package when built from source by CMake with the appropriate config option set. This means that make docs fails for MlirEnv.

To Reproduce

Steps to reproduce the behavior:

1. make docs
2. Observer AttributeError: MlirEnv error:

WARNING: autodoc: failed to import class 'MlirEnv' from module 'compiler_gym.envs'; the following exception was raised:
Traceback (most recent call last):
  File "/private/home/cummins/.conda/envs/compiler_gym/lib/python3.8/site-packages/sphinx/util/inspect.py", line 412, in safe_getattr
    return getattr(obj, name, *defargs)
AttributeError: module 'compiler_gym.envs' has no attribute 'MlirEnv'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/private/home/cummins/.conda/envs/compiler_gym/lib/python3.8/site-packages/sphinx/ext/autodoc/importer.py", line 110, in import_object
    obj = attrgetter(obj, mangled_name)
  File "/private/home/cummins/.conda/envs/compiler_gym/lib/python3.8/site-packages/sphinx/ext/autodoc/__init__.py", line 318, in get_attr
    return autodoc_attrgetter(self.env.app, obj, name, *defargs)
  File "/private/home/cummins/.conda/envs/compiler_gym/lib/python3.8/site-packages/sphinx/ext/autodoc/__init__.py", line 2606, in autodoc_attrgetter
    return safe_getattr(obj, name, *defargs)
  File "/private/home/cummins/.conda/envs/compiler_gym/lib/python3.8/site-packages/sphinx/util/inspect.py", line 428, in safe_getattr
    raise AttributeError(name) from exc
AttributeError: MlirEnv

Environment

Please fill in this checklist:

• CompilerGym: v0.2.3
• How you installed CompilerGym (pip, source): source
• OS: Linux
• Python version: 3.8