Make InMemImageReader work even when monai is not used/available

[gigony]

If the user's application doesn't depend on monai and monai is not installed, it causes an error when the following statement is executed by the interpreter.

monai-deploy-app-sdk/monai/deploy/operators/monai_seg_inference_operator.py

Line 239 in eebb284

class InMemImageReader(ImageReader):

This patch sets ImageReader to object if monai library is not available so the user's app doesn't require monai app if not using.

[MMelQin]

The InMemImageReader is a derived class of MONAI ImageReader, hence dependency on MONAI be present.

This PR is wrong trying to break this dependency at runtime, and should be rejected.

[gigony]

The InMemImageReader is a derived class of MONAI ImageReader, hence dependency on MONAI be present.

This PR is wrong trying to break this dependency at runtime, and should be rejected.

@MMelQin , existing code caused an error even when user's application doesn't use monai.

I faced the issue somehow and
I think you can reproduce it with simple_imaging_app in a fresh python virtual environment.

Let me also double check it in case the PR cause the side effect you described.

Thank you!

[MMelQin]

The example you mentioned should not be loading the monai_seg_inference_operator classes at all, where the InMemImageReader is. So puzzled by the error you saw, while setting the reader to object in the seg operator is not the way to address the issue, even if it can help to avoid the symptom (optional_import was already used in the code).