Nmnist fix

[timcheck]

Issue Number: 57

Objective of pull request: Increased GPU utilization on N-MNIST tutorial

Pull request checklist

Your PR fulfills the following requirements:

• Issue created that explains the change and why it's needed
• Tests are part of the PR (for bug fixes / features)
• Docs reviewed and added / updated if needed (for bug fixes / features)
• PR conforms to Coding Conventions
• PR applys BSD 3-clause or LGPL2.1+ Licenses to all code files
• Lint (flakeheaven lint src/lava tests/) and (bandit -r src/lava/.) pass locally
• Build tests (pytest) passes locally

Pull request type

Please check your PR type:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation changes
• Other (please describe):

What is the current behavior?

• GPU utilization is not high enough.

What is the new behavior?

• GPU utilization hits >50%.

Does this introduce a breaking change?

• Yes
• No

Supplemental information

The user may need to adjust num_worker threads according to the capabilities of the user's CPU. Likewise, the user may need to reduce the batch size to fit in the user's GPU memory.

[bamsumit]

Hi, @timcheck can you complete this PR?