[onert/python] Separate prepare from session constructor

[ragmani]

This commit separates prepare function from initializing session instance.

ONE-DCO-1.0-Signed-off-by: ragmani [email protected]

[ragmani]

For #14505
Draft : #14492

[hseok-oh]

Isn't it better to introduce new python API for prepare function to split with __init__ function?

[ragmani]

Isn't it better to introduce new python API for prepare function to split with init function?

I also think it's better, but it makes the python API changes for inference. Is it OK?
From:

    # Create session and load nnpackage
    # The default value of backends is "cpu".
    session = infer.session(nnpackage_path, backends)
    ...
    outputs = session.inference()

To:

    # Create session and load nnpackage
    # The default value of backends is "cpu".
    session = infer.session(nnpackage_path, backends)
    session = infer.prepare()
    ...
    outputs = session.inference()

[hseok-oh]

Considering easy usage, we can remain current API - prepare at once if session is created with file path, and support new feature to create session only when file path is not passed and support load API and prepare API separately.

[hseok-oh]

@ragmani Please resolve conflict

[ragmani]

Considering easy usage, we can remain current API - prepare at once if session is created with file path, and support new feature to create session only when file path is not passed and support load API and prepare API separately.

I added the code to allow the use of Prepare in the draft 8ef5277. If you feel it's acceptable, I will apply it to the next PR.

[ys44kim]

LGTM

[zetwhite]

LGTM

[hseok-oh]

LGTM