🐛[Fix] fix validator to handle custom datasets with txt files

[DizzyZff]

Description

When the input validation dataset is using txt labels the Validator throws exception on these lines:

https://github.com/WongKinYiu/YOLO/blob/8228669808a626fc5f9c233fdb35550b5e041fae/yolo/tools/solver.py#L231-L233

Add a condition for txt files to bypass this if.

Closes #94

Type of Change

Please delete options that are not relevant.

• 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:

• The code follows the Python style guide.
• Code and files are well organized.
• All tests pass.
• New code is covered by tests.
• The pull request is directed to the corresponding topic branch.
• [Optional] We would be very happy if gitmoji 🧑‍💻 could be used to assist the commit message 💬!

Licensing:

By submitting this pull request, I confirm that:

• My contribution is made under the MIT License.
• I have not included any code from questionable or non-compliant sources (GPL, AGPL, ... etc).
• I understand that all contributions to this repository must comply with the MIT License, and I promise that my contributions do not violate this license.
• I have not used any code or content from sources that conflict with the MIT License or are otherwise legally questionable.