refactor imports of optional dependencies

[Borda]

What does this PR do?

drop tricky try/catch. form optional imports, @awaelchli
resolves #2266

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[Borda]

@awaelchli any idea how it is possible that the package is found so XXXX_AVAILABLE is true. but the import fails?

[Borda]

@awaelchli it seems that the conditional import does not work, it is imported even inside the if statement... :/

[akihironitta]

@Borda Seems _module_available needs some fix.
I ran the following script:

import importlib

def _module_available(module_path: str) -> bool:
    """Testing if given module is avalaible in your env
    >>> _module_available('system')
    True
    >>> _module_available('bla.bla')
    False
    """
    mods = module_path.split('.')
    assert mods, 'nothing given to test'
    # it has to be tested as per partets
    for i in range(1, len(mods)):
        module_path = '.'.join(mods[:i])
        if importlib.util.find_spec(module_path) is None:
            return False
    return True

print(_module_available("asdf"))
print(_module_available("asdf.asdf"))

and encountered the following unexpected behaviour...

True
False

[Borda]

@Borda Seems _module_available needs some fix.

Great catch, I am adding also some more test for the func... fix in #4871

[Borda]

@tgaddair mind have a look at why the Horovod tests are failing?

[Borda]

after merging all the partial PRs this remain only for Horovod

[codecov]

Codecov Report

Merging #4859 (b88d6f2) into master (02ac3ab) will decrease coverage by 0%.
The diff coverage is 80%.

@@          Coverage Diff           @@
##           master   #4859   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         124     124           
  Lines        9302    9295    -7     
======================================
- Hits         8615    8608    -7     
  Misses        687     687           

[Borda]

the TPU and GPU #4884 failing is unrelated to this PR

RuntimeError: Failed precondition: From /job:tpu_worker/replica:0/task:0:
2 root error(s) found.
  (0) Failed precondition: Expected comparison type FLOAT or TOTALORDER.
actual: UNSIGNED
operand: f32[32,10]

	 [[{{node XRTCompile}}]]
  (1) Failed precondition: Expected comparison type FLOAT or TOTALORDER.
actual: UNSIGNED
operand: f32[32,10]

	 [[{{node XRTCompile}}]]
	 [[XRTCompile_G6]]

cc: @zcain117

[tchaton]

LTGM !