Enable to follow symbolic link directories when dataset upload

[i-aki-y]

In this PR, I want to allow following symbolic link directories when uploading a dataset.

The current implementation looks to silently ignore symbolic link directories when upload a dataset because of os.walk's default behavior.
This can confuse the user. In particular, it is possible that missing only a portion of the dataset will cause a slight mismatch in the model's performance between trained by the original and uploaded datasets. Identifying the cause of such problems can be difficult.

So I want to add a flag that allows following symbolic link directories and add some log messages that inform for user when some directories are skipped.

related issue? #119

I would appreciate it if you could review this.

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[yitang]

useful feature to have