Turn command import into optional

[JingyaHuang]

What does this PR do?

as per title.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[fxmarty]

I am personally not in favor of this - basically some commands will not show depending on whether you have installed a package or not? Ideally you would want to see all possible commands, with errors raised if you miss a package for one.

[JingyaHuang]

@michaelbenayoun WDYT?

@fxmarty What about my initial suggestion on not setting any default subcommand, and add them by register? Like registering onnx, tflite and onnxruntime in a register.py and in optimum-neuron override it with a same register.py but only register neuron associated commands. In this way when using the cli of optimum message will be related to submodule in optimum main, and in neuron, it will only be neuron related requirements. (although they are all optimum-cli, IMO it's fine not poping out onnx info whenusing optimum-cli under neuron env)

[michaelbenayoun]

I agree with @fxmarty, what about doing this instead: each BaseOptimumCliCommand can define a check_requirements method, that is being run at in the right time, to fail if the command is ran without the requirement, and nothing happens otherwise?

[JingyaHuang]

Can I merge this one? @michaelbenayoun @fxmarty

[michaelbenayoun]

LGTM (as long as the tests pass)

[fxmarty]

LGTM, thanks a lot!