Improvements to assessment colab.

- Label CSV files are now be uploaded into the same directory where labeled
  examples are stored. These directories are automatically created and
  timestamped, so multiple versions of the label CSV can be uploaded without
  overwriting previous versions. Label CSVs are also uploaded in the same step
  as labeled example creation, not in a separate step.

- Inference CSVs are generated in the same directory where the model is
  stored. Inference CSV filename can also be specified by the user.

- Model inference now runs on GPU workers. This wasn't supported previously
  because colab's python version wasn't 3.11, the version needed to be
  compatible with GPU containers.

PiperOrigin-RevId: 722809148