fix: allow S3 folder input to contain a trailing slash in Local Mode

[laurenyu]

Issue #, if available:

Description of changes:
If one creates an S3 folder in the AWS console, it creates both a folder and an object (because "S3 doesn't have folders"). As a result, our logic for downloading a folder (see #1295) would fail to correctly download the contents of a folder if the object was present and the S3 prefix was given with a trailing slash.

Testing done:

• unit tests
• tested manually using the PyTorch MNIST notebook
• also manually verified that ListBucket Permission #1283 is still resolved

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