[FEATURE] Feature new IMG_EXTENSIONS ".tif" in timm/data/parsers/constants.py 

Hello.

**Is your feature request related to a problem? Please describe.**
I have some images with the ".tif" extension and thus I get "RuntimeError: Found 0 images in subfolders of ... Supported image extensions are .png, .jpg, .jpeg".

**Describe the solution you'd like**
Add ".tif" in "timm/data/parsers/constants.py"

Thanks for your work and this amazing github.