Remove embedding_dim parameter in ImageEmbedder examples

[tszumowski]

What does this PR do?

What

• Removed embedding_dim parameter in the ImageEmbedder README example
• Removed embedding_dim parameter in the ImageEmbedder code example
• Searched for all other references to embedding_dim usgae -- Only other one is in a unit test, which should stay

Why

• Reference: Readme and example proposes to use custom embedding size for ImageEmbedder, but that is invalid according to the code warnings #653
• According to code warning, embedding_dim should only be used if the user fine-tunes after. That is not done in these examples, and probably shouldn't anyway since that's a bit out of scope for the simple example.

Fixes # (issue)

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements) -- Readme and example proposes to use custom embedding size for ImageEmbedder, but that is invalid according to the code warnings #653
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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[tszumowski]

Uh oh. I now realize that you may need to flatten that embedding for that resnet example. Because IIRC, resnet returns a 2048x7x7 output. Will put into draft mode for now, and take it out of draft when ready with a test result.

[tszumowski]

I found a potential error. If you run without embedding_dim assigned, it takes the last layer of the backbone. That's not always flattened. For example, in Resnet101, it's 2048x7x7. That's not compatible with embedding applications without flattening.

I'll close this PR for now and discuss this in a separate issue.

[tszumowski]

Discussion migrated to:
#656