Update estimator docstrings to add Fast File Mode

Author: shreyapandit

src/sagemaker/estimator.py

@@ -153,9 +153,10 @@ def __init__(
                 (default: 'File'). Valid modes: 'File' - Amazon SageMaker copies
                 the training dataset from the S3 location to a local directory.
                 'Pipe' - Amazon SageMaker streams data directly from S3 to the
-                container via a Unix-named pipe. This argument can be overriden
-                on a per-channel basis using
-                ``sagemaker.inputs.TrainingInput.input_mode``.
+                container via a Unix-named pipe.
+                'FastFile' - Amazon SageMaker streams data from S3 on demand instead of
+                downloading the entire dataset before training begins. This argument can be overriden
+                on a per-channel basis using ``sagemaker.inputs.TrainingInput.input_mode``.
             output_path (str): S3 location for saving the training result (model
                 artifacts and output files). If not specified, results are
                 stored to a default bucket. If the bucket with the specific name