Add tf.BenchmarkConfig that returns a session config appropriate for...

Add tf.BenchmarkConfig that returns a session config appropriate for benchmarking. At the moment, it returns a default config with only Grappler dependency optimizer disabled. Many benchmarks wrap the subgraph they want to time in control_flow_ops.group() to avoid including the overhead of copying the output back to the Python client in the measurement. In the graph, this only adds a control dependency between the subgraph output and the fetch node, which in turn (often) causes the dependency optimizer to turn all nodes in the graph into no-ops.

PiperOrigin-RevId: 216242463