:bug: Fix Assertion error - darknet .weights to .h5

[DaemonOnCode]

Description

• google colab specific assertion issue caused while converting darknet weights to h5 format
• loading custom yolov4.h5 weights by locating the correct directory

Fixes #72

Type of change

Please delete options that are not relevant.

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

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented on my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

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[DaemonOnCode]

Respected @RaphaelMeudec @AntoineToubhans @lmontier could you please review my pull request and merge. I have checked and run locally. Also, you could view the ReviewNB to check the change. This fixes an issue a lot of people are having.