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Get a partitioner for VariableScope to keep shards below max_shard_bytes.

This partitioner will shard a Variable along one axis, attempting to keep the maximum shard size below max_shard_bytes. In practice, this is not always possible when sharding along only one axis. When this happens, this axis is sharded as much as possible (i.e., every dimension becomes a separate shard).

If the partitioner hits the max_shards limit, then each shard may end up larger than max_shard_bytes. By default max_shards equals None and no limit on the number of shards is enforced.

One reasonable value for max_shard_bytes is (64 << 20) - 1, or almost 64MB, to keep below the protobuf byte limit.

max_shard_bytes The maximum size any given shard is allowed to be.
axis The axis to partition along. Default: outermost axis.
bytes_per_string_element If the Variable is of type string, this provides an estimate of how large each scalar in the Variable is.
max_shards The maximum number of shards in int created taking precedence over max_shard_bytes.

A partition function usable as the partitioner argument to variable_scope and get_variable.

ValueError If any of the byte counts are non-positive.