fix: skip local file check for TF requirements file when source_dir is an S3 URI (#798)

Author: imujjwal96

src/sagemaker/tensorflow/estimator.py

@@ -283,6 +283,9 @@ def _validate_requirements_file(self, requirements_file):
         if not self.source_dir:
             raise ValueError('Must specify source_dir along with a requirements file.')
 
+        if self.source_dir.lower().startswith('s3://'):
+            return
+
         if os.path.isabs(requirements_file):
             raise ValueError('Requirements file {} is not a path relative to source_dir.'.format(
                 requirements_file))