Refactor main functions to enable multi-node distribution

[francois-drielsma]

Description

This PR enables multi-node distributed training (proper handling of externally set RANK/WORLD_SIZE.

Type of Change

This is a new, non-breaking feature:

• New feature (non-breaking change which adds functionality)
• Code quality improvement (refactoring, type hints, etc.)
• Performance improvement

How Has This Been Tested?

I have trained UResNet-PPN with single-node/single-gpu, single-node/multi-gpu and multi-node/multi-gpu settings, all function as expected. See /sdf/data/neutrino/ndlar/spine/train/multinode.

Test Configuration:

• SPINE version: v0.10.5

[codecov]

Codecov Report

❌ Patch coverage is 22.00000% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 15.01%. Comparing base (db8e20a) to head (29c1b02).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spine/main.py	29.16%	17 Missing ⚠️
src/spine/model/manager.py	15.38%	11 Missing ⚠️
src/spine/utils/torch/devices.py	22.22%	7 Missing ⚠️
src/spine/bin/cli.py	0.00%	3 Missing ⚠️
src/spine/driver.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
- Coverage   15.17%   15.01%   -0.17%     
==========================================
  Files         273      273              
  Lines       20572    20591      +19     
==========================================
- Hits         3122     3091      -31     
- Misses      17450    17500      +50     

Flag	Coverage Δ
unittests	15.01% <22.00%> (-0.17%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/spine/driver.py	11.14% <0.00%> (ø)
src/spine/bin/cli.py	22.58% <0.00%> (-0.56%)	⬇️
src/spine/utils/torch/devices.py	22.72% <22.22%> (+2.72%)	⬆️
src/spine/model/manager.py	10.38% <15.38%> (-8.73%)	⬇️
src/spine/main.py	18.05% <29.16%> (-0.70%)	⬇️

... and 3 files with indirect coverage changes

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