Add feature to optionally provide stats to trainer so number of examples can be determined dynamically

[pritamdodeja]

Enables the user to use number of examples information computed by StatisticsGen in their training code. Passing statistics to trainer enables the use of

splits = fn_args.num_examples.keys()                                                                                                                                                                  
training_examples = fn_args.num_examples['train']                                                                                                                                            
validation_examples = fn_args.num_examples['eval']

in

run_fn(fn_args: tfx.components.FnArgs):

The configuration of Trainer would look as follows:

trainer = Trainer(                                                                                                                                                                                          
    module_file=trainer_file,                                                                                                                                                                               
    examples=transform.outputs['transformed_examples'],                                                                                                                                                     
    transform_graph=transform.outputs['transform_graph'],                                                                                                                                                   
    schema=transform.outputs['post_transform_schema'],                                                                                                                                                      
    hyperparameters=tuner.outputs['best_hyperparameters'],                                                                                                                                                  
    statistics=statistics_gen.outputs['statistics'], #new
)

More details at:
#7700