Memory requirement spike for `gandlf_preprocess`

[sarthakpati]

Describe the bug When gandlf_process is run (with normalize and crop_external_zero_plane as preprocessing parameters), the process runs fine for a validation csv with ~180 subjects, but fails with OOM error for training csv with ~800 subjects using the BraTS data.

To Reproduce

1. Construct a CSV of ~180 BraTS subjects, and another with ~720 subjects (copy-pasting the same 180 cases with different subject IDs should reproduce the error).
2. Construct a config (model and training parameters don't matter) but have this key for preprocessing:

   data_preprocessing:
   {
   'normalize_nonZero',
   'crop_external_zero_planes',
   }
   patch_sampler:
   {
   'type': 'label', 
   } 

3. Run the gandlf_preprocess script for both these cases.
4. See it pass for the one with ~180 cases and fail for ~720 cases. This is with 250G RAM.

Expected behavior It should run for both.

Screenshots N.A.

GaNDLF Version

0.0.18-dev

Desktop (please complete the following information): N.A.

Additional context Memory profiler (thanks @hasan7n): https://pypi.org/project/memory-profiler/

[VukW]

Is the corresponding BraTS data publicly available? Can you provide it also, please?

[sarthakpati]

You should be able to download the data here: https://www.synapse.org/brats

And this should be replicable even on the unit testing data [ref].

Do you think you can include the report from #806 into your fix as well (since both related to memory consumption)?

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[sarthakpati]

This is still under investigation.

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