Fix problematic header read/write for *.weights file

[zjuyzj]

Proposed changes

In the latest version 1.8.0, Darknet.save_darknet_weights() generates a problematic *.weights file, in which "minor" field is always 0, however the length of "seen" field is 8 bytes.

This may cause abnormal offset calculation when try to use official Darknet or other third-party Darknet-compatible repository (e.g. rknn-toolkit by Rockchip) to load the exported *.weights file, corrupting the whole weights and biases behind the header, then leading to totally wrong inference!

Reference to the read and write of *.weights header, from Darknet's official implementation:

• void load_weights_upto(network *net, char *filename, int start, int cutoff)
  (https://github.com/pjreddie/darknet/blob/master/src/parser.c#L1218)
• void save_weights_upto(network *net, char *filename, int cutoff
  (https://github.com/pjreddie/darknet/blob/master/src/parser.c#L1007)

Related issues

Resolves #292, and all other problems likewise caused by wrong exported *.weights are totally resolved.

However, saved module state_dict (*.pth file) does not contain the value of "seen" class member, which is 0 by default.
Convert a *.pth to *.weights will fill the "seen" field zero, losing the original "seen" from any pretrained *.weights, which is just a little problem and not related to inference BTW.

Necessary checks

• Update poetry package version semantically
• Write documentation
• Create issues for future work
• Test on your machine