to-tf-od: rework shard output

[fracpete]

Currently, the -s option allows one to specify the shard files to generate. However, this is not feasible when creating hundreds of shard files.
Instead have the following two options:

• --num_shards <int>: how many shard files to generate, default is -1, so no sharding
• --format <format>: the format for the file names (see below)
• --width <int>: the number of digits to use to the index/total number of shards in the file name (left-padded with 0s), default is 5

The shard format string supports the following two placeholders:

• {INDEX} - the 1-based index for the shard file
• {TOTAL} - the total number of shard files that get generated (see --num_shards)

This allows the specification of formats:

• --format "train.record-{INDEX}-{TOTAL}" (see here)
• --format "train-{INDEX}-{TOTAL}.tfrecords"