BF: Add block_length param to dataset.interleave

[richford]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Summary

Resolves #174
This PR adds a block_length param to the dataset.interleave method. block_length should be set to the examples_per_shard that were used to write the original TFRecords dataset. Without this, get_data inadvertently shuffles its input data.

Checklist

• I have added tests to cover my changes
• I have updated documentation (if necessary)

Acknowledgment

• I acknowledge that this contribution will be available under the Apache 2 license.

[richford]

I realize that you're waiting on me to add more unit tests for #177. Is there anything that I'm missing for this PR?