TrainerConfig inherit from InstantiateConfig (#1175)

Making TrainerConfig an InstantiateConfig so it can be swapped in and out with different methods. Also made the TrainerConfig a subclass of ExperimentConfig (which was changed from a printableconfig to an instantiateconfig), since we were already instantiating the Trainer class with the ExperimentConfig

Removed the "config" field from the TrainerCallbackAttributes struct to eliminate circular dependencies when types are checked.

Minor scripts and docs changes to accommodate the update to the config struct, meaning the flags that were associated with the TrainerConfig were moved up to the top level of the config.

docs/quickstart/first_nerf.md

@@ -35,7 +35,7 @@ Navigating to the link at the end of the terminal will load the webviewer. If yo
 It is possible to load a pretrained model by running
 
 ```bash
-ns-train nerfacto --data data/nerfstudio/poster --trainer.load-dir {outputs/.../nerfstudio_models}
+ns-train nerfacto --data data/nerfstudio/poster --load-dir {outputs/.../nerfstudio_models}
 ```
 
 This will automatically start training. If you do not want it to train, add `--viewer.start-train False` to your training command.