Rm dep on tf.contrib.data.LMDBDataset for TF 2.0

[rsepassi]

tf.contrib.data.LMDBDataset will not make it to TF2. It may be moving to a new repo/package. If/when that happens, we should switch to it.

Part of #31 (TF 2.0 support)

[yongtang]

LMDB has been migrated to tensorflow/io (tensorflow/io#74). The PR has been merged into the master and should be available in next release soon (hopefully in a couple of weeks).

[rsepassi]

Ok, thank you @yongtang. Could you update us when it's available in the release? TFDS tries to support Python 2 and Python 3 as well as TF 1.13, nightly, and 2.0. Will we be able to use your package (i.e. will it work across that matrix)?

[ParthS007]

@rsepassi tensorflow/io has the latest release 20 days ago and LMDBDataset is also included in the release when I looked at the source code of release.
I want to work on this Issue. Can you please assign this to me and help me to go ahead with this issue. Thanks :)

[rsepassi]

Hi @ParthS007, thank you! Please see the docs here to add the dep to LSUN.

[vijayphoenix]

This issue seems to be solved by ec89c3e.
Corresponding PR #468 can also be closed.