Torch>=1.8 only, better beamforming and MDX challenge baseline

⚠️ This release drops support for PyTorch under 1.8 and restricts PyTorch-Lightning under 1.3.
The next release (0.5.1) will add support to PyTorch-Lightning 1.3.0 which broke our CI. ⚠️

Highlights

• Beamforming modules are now more stable (computed in double by default), have been renamed and extended.
• The baseline for the Music Demixing Challenge has been released 🎉

Changelog

Breaking

• [src&tests] Remove all deprecated code (#474)
• [all] Drop support for torch<1.8.0 (#476)

Added

• [src] Beamforming: Souden MVDR and optimal channel selection (#484)
• [src&egs] X-UMX Official baseline for Music Demixing Challenge (#490)

Changed

• [src] Compute linalg beamforming ops in double (#482)
• [src] Improve Beamforming naming and add TODOs (#483)
• [src] Beamforming: enable forcing float linalg ops (#485)
• [docs] Update instructions for pretrained model sharing (#489)
• [install] Bump lightning version under 1.3.0 (#493)

Fixed

• [nb] Fix shape in 00_GettingStarted.ipynb (#478)
• [src] Stabilize GEV beamformer (#479)
• [src] Beamforming: fix doc references

Thanks to @quancs @r-sawata @popcornell and @faroit for their contributions