Returns metrics and plots writer.

Note, sharding will be enabled by default if a output_file_format is provided. The files will be named -SSSSS-of-NNNNN. where SSSSS is the shard number and NNNNN is the number of shards.

output_paths Output paths keyed by output key (e.g. 'metrics', 'plots', 'validation').
eval_config Eval config.
add_metrics_callbacks Optional list of metric callbacks (if used).
metrics_key Name to use for metrics key in Evaluation output.
plots_key Name to use for plots key in Evaluation output.
validations_key Name to use for validations key in Evaluation output.
output_file_format File format to use when saving files. Currently only 'tfrecord' is supported.