Add pixelNeRF to the repository

[mihaimorariu]

The pixelNeRF paper will be added to the repository: https://arxiv.org/abs/2012.02190

Tasks to be completed

• [x] Implement DTU dataset loader
• [x] Implement the encoder
• [x] Implement the NIF model
• [x] Implement the renderer
• [x] Implement the pipeline
• [x] Implement the losses
• [x] Write tutorial on how to use the code
• [ ] Review

Definition of Done

• [x] The results are reproduced
• [x] Training, evaluation scripts are provided
• [x] Tutorial is provided