Tidying up Train.ipynb, README, and accel.slurm

[PaulScotti]

• Fixed README instructions, you don't need to download ALL of the huggingface dataset
• Switched from conda env to venv (in line with Stability HPC guidance)
• Tidied up src directory, moving extra notebooks into a folder and going back to having a simple design of a base notebook + models.py + utils.py
• MLPMixer seq_len mechanism now working properly
• Rather than feeding the averaged voxels across the same-image repeats through the model once, validation testing now involves the feeding each single-image sample through model separately and averaging across those outputs. This is important as without doing this it leads to complete failure to use "memory" (aka seq_len > 1)
• Removed depth estimations for now to keep things simpler for onboarding
• Using SDXL VAE for blurry reconstructions
• Can now set num_sessions and seq_len as argparse arguments
• accel.slurm example for sbatch now will convert the Train.ipynb file to .py automatically
• Removed deepspeed for now to keep things simpler for onboarding
• Removed cells after the model training to keep things simpler for onboarding (can still reference the notebooks in more_notebooks/ folder)