Allow custom input resolution.

[MartinBrazdil]

Hi, thank you for this wonderful repository :-)

Here is my bit which allows inference with custom input resolution (e.g. 720x1280).

BTW I have also fixed score threshold in the same way as in this pull request.

I have not tested training yet but hopefully it would work.

[pep8speaks]

Hello @MartinBrazdil! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file demo.py:

Line 41:80: E501 line too long (97 > 79 characters)
Line 47:80: E501 line too long (90 > 79 characters)

• In the file models/bifpn.py:

Line 192:80: E501 line too long (107 > 79 characters)