Add the OverfitModel

Summary:
Introduces the OverfitModel for NeRF-style training with overfitting to one scene.
It is a specific case of GenericModel. It has been disentangle to ease usage.

## General modification

1. Modularize a minimum GenericModel to introduce OverfitModel
2. Introduce OverfitModel and ensure through unit testing that it behaves like GenericModel.

## Modularization

The following methods have been extracted from GenericModel to allow modularity with ManyViewModel:
- get_objective is now a call to weighted_sum_losses
- log_loss_weights
- prepare_inputs

The generic methods have been moved to an utils.py file.

Simplify the code to introduce OverfitModel.

Private methods like chunk_generator are now public and can now be used by ManyViewModel.

Reviewed By: shapovalov

Differential Revision: D43771992

fbshipit-source-id: 6102aeb21c7fdd56aa2ff9cd1dd23fd9fbf26315

projects/implicitron_trainer/configs/overfit_singleseq_base.yaml

@@ -0,0 +1,42 @@
+defaults:
+- overfit_base
+- _self_
+data_source_ImplicitronDataSource_args:
+  data_loader_map_provider_SequenceDataLoaderMapProvider_args:
+    batch_size: 1
+    dataset_length_train: 1000
+    dataset_length_val: 1
+    num_workers: 8
+  dataset_map_provider_JsonIndexDatasetMapProvider_args:
+    assert_single_seq: true
+    n_frames_per_sequence: -1
+    test_restrict_sequence_id: 0
+    test_on_train: false
+model_factory_ImplicitronModelFactory_args:
+  model_class_type: "OverfitModel"
+  model_OverfitModel_args:
+    render_image_height: 800
+    render_image_width: 800
+    log_vars:
+    - loss_rgb_psnr_fg
+    - loss_rgb_psnr
+    - loss_eikonal
+    - loss_prev_stage_rgb_psnr
+    - loss_mask_bce
+    - loss_prev_stage_mask_bce
+    - loss_rgb_mse
+    - loss_prev_stage_rgb_mse
+    - loss_depth_abs
+    - loss_depth_abs_fg
+    - loss_kl
+    - loss_mask_neg_iou
+    - objective
+    - epoch
+    - sec/it
+optimizer_factory_ImplicitronOptimizerFactory_args:
+  lr: 0.0005
+  multistep_lr_milestones:
+  - 200
+  - 300
+training_loop_ImplicitronTrainingLoop_args:
+  max_epochs: 400