Edits to TF 3D Unet BraTS workspace

[mansishr]

Some fixes to 'tf_3dunet_brats` workspace:

1. Pinning down TF version to 2.8 for this tutorial. The support for optimizer.get_weights() has been removed in the recent versions of TF.
2. Update documentation on running hello_federation script and creating softlinks for data directory.
3. Remove num_batches from the plan, since train_iteration is able to generate batches from the DL itself.