TF2 environment settings

[dakshvar22]

Proposed changes:

• Refactor how tensorflow enviornment is configured for tensorflow 2.0
• Add environment variables, example shown below:

TF_GPU_MEMORY_ALLOC="0:1024, 1:2048" # This specifies to use 1024 MB of memory from GPU with logical ID 0 and 2048 MB of memory from GPU with logical ID 1"

TF_INTER_OP_PARALLELISM_THREADS="3" # Specifies that atmost 3 CPU threads can be used to parallelize multiple non-blocking operations

TF_INTRA_OP_PARALLELISM_THREADS="2" # Specifies that atmost 2 CPU threads can be used to parallelize a particular operation.

part of https://github.com/RasaHQ/research/issues/62

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[Ghostvv]

we need to document it, also there are old tf_config tests, that need to be removed (unless they're already removed)

[wochinge]

Thanks for also writing comprehensive docs for this 🚀

[wochinge]

Added two more comments, but we are ready to go now. Feel free to merge after checking the comments! 🥇