[Task] Update notebooks to note we require TF 2.8+ in the notebooks.

[benfred]

Description

The notebooks currently include code like

import os
os.environ["TF_GPU_ALLOCATOR"]="cuda_malloc_async"
import merlin.models.tf as mm

This causes a segfault in TF 2.6. We should add a note to our notebooks saying that this requires TF 2.8+

[EvenOldridge]

And instructions commented out for how to deal with this in 2.6 and lower.

[rnyak]

related to https://github.com/NVIDIA-Merlin/Merlin/issues/229

[rnyak]

Based on Benedikt's experiment on testbook unit tests, he could avoid Jenkins error after removing `os.environ["TF_GPU_ALLOCATOR"]="cuda_malloc_async" line.

[viswa-nvidia]

@benfred , do we need to track this still?

[bschifferer]

@viswa-nvidia I closed the ticket, as there is no progress. Please reopen, if we require it.