Support outputting HLO in leela2onnx

[mooskagh]

In addition to (now optional) --output= parameter of leela2onnx,

• --hlo-text-output=filename to output as text HLO (or stdout if filename is -)
• --hlo-proto-output=filename to output to binary HloModuleProto file
• --hlo-batch-size -- for which batch size to generate HLO (333 is the default, to be noticeable in the output file)
• --hlo-allow-partial-result -- if conversion fails (i.e. due to ONNX op not being implemented), still output what was converted so far. Useful to look at the progress while implementing new ops.

[borg323]

xla_hlo_builder.cc needs a #include <unordered_map> print_hlo.cc needs a #include <cctype>