Only load from model repository if model binary is not found under model_dir

[sivanantha321]

What this PR does / why we need it:
Previously, the model repository is always initialized, and searches for the models to load from the model_dir. This is unnecessary, if we only have a single model under model_dir.
This PR includes the changes for how the model is loaded.

• Assumes the model_dir contains a single model binary in the parent directory and tries to load the model.
  Here the model dir structure would be

model_dir/
       model.bin     # model_binary

• If the model load failed, Initialize the model repository and try to load model/models from the repository.
  Here the model dir structure would be

model_dir/
    some_model_1/
          model_1.bin  # model binary

    some_model_2/
          model_2.bin  # model binary
    

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Type of changes
Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Feature/Issue validation/testing:

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
Please also list any relevant details for your test configuration.

• Test A

• Test B

• Logs

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Checklist:

• Have you added unit/e2e tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

Release note:

Only load from model repository if model binary is not found under model_dir

[terrytangyuan]

/lgtm

[yuzisun]

/approve

[oss-prow-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sivanantha321, terrytangyuan, yuzisun

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