Decide how to handle str vs bytes dtype input

[onishchenkod]

So far default dtype to fir qnets on is string,
however bytestrings offer better memory efficiency that proved to be crucial in recent applications.

When qnet instance is fit on bytestrings, its outputs for qsampling etc. are in bytestrings as well, so this matters

I see two options:
1 - make qnets support both strings and bytestrings simultaneously.
2 - specify the accepted dtype at the creation of qnet instance and make it only accept the selected dtype input.