Parallel directory iterator initialization (#6890)
Parallel directory iterator initialization (#6890)

* added parallel counting of sample files when initializing DirectoryIterator

* Updated to actually run in parallel.

* Added parallel generation of the filenames and labels lists

* Added documentation and removed commented-out code

* style fixes

* changes discussed in pull request

* Removed trailing spaces

* Switching to thread pool

* fixed broken import

keras-team/keras,keras-team/keras