Making deep learning backends truly optional at import time

[C-Achard]

Right now, importing DLCLive also imports torch/tf in several places without guards.

Examples

Examples include :

• benchmark.py
• live_inference.py
• ...

Whereas certain modules such as utils.py properly guard the imports.

Motivation

Properly guarded imports and ideally introducing "lazier" behavior would drastically help in making truly lightweight tests in CI, and likely improve import times drastically.