Added extension as an option when saving the result images.

[samet-akcay]

Description

• This PR converts bmp images to png after downloading and extracting the BTech dataset.
  Each BTech category has different image extension as follows

Category	Image	Mask
01	bmp	png
02	png	png
03	bmp	bmp

To avoid any conflict, the following script converts all the extensions to png. This solution works fine, but it's also possible to properly ready the bmp and png filenames from categories in make_btech_dataset function.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the pre-commit style and check guidelines of this project.
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes