Remove Unused Functions in apex\apex\transformer\tensor_parallel\random.py

[syedshazli]

In random.py, there are several functions which have a TODO to remove the functions due to being unused by megatron-lm. However, these functions still have not been removed.

In this PR, I removed the unused functions named 'init_checkpointed_activations_memory_buffer' and 'reset_checkpointed_activations_memory_buffer', as well as the global variable '_CHECKPOINTED_ACTIVATIONS_MEMORY_BUFFER'

[syedshazli]

cc @crcrpar