fast vector operation for pillar scatter

[dmund95]

This PR introduces fast vector operations for pillar scatter module. The for loop in exisiting module make model forward very slow. Especially with larger batch sizes.

The code has been tested to check for equal outputs before and after the changes and included in this PR

latency experiments:

1. forward times as a function of batch_size

2. Overall training time before and after the change

Before -> Average per iteration training time ~2.9sec

After -> Average per iteration training time ~1.9sec

The training time per iteration reduces by 35% with this PR (for my set of parameters / dataset)

[Petros626]

@dmund95 the pipeline stayed the same and training is working for your properly? Seems like an useful improvement.

[dmund95]

@dmund95 the pipeline stayed the same and training is working for your properly? Seems like an useful improvement.

Yes, I have been training and deploying models with this change and no issues.