Adapt patch_extract function for rgba imgs in train_kmeans.py

[jksshack]

Describe your changes
Adds an elif statement that addresses rgba images
The alpha channel is removed before applying gaussian blur

Type of update

• Bug fix

Associated issues
#1749

Additional context

For the reviewer
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• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
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[deepsource-io]

Here's the code health analysis summary for commits 83b15e0..e962319. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	❌ Failure	❗ 1 occurence introduced 🚨 1 failing metric	View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	100%	100%
Line Coverage	100%	100%
New Branch Coverage	100%	100%
New Composite Coverage	0%	0%
New Line Coverage	0%, ❌ Below Threshold	0%, ❌ Below Threshold

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