Improve pidTPCML.cxx

[ktf]

• Out of line code to reduce memory usage while compiling pidTPC tasks
• Change to use an array for the intermediate tracks representation.
  It would probably be better to read directly the values from the
  arrow tables, but that will come in a different PR.

[ktf]

@njacazio @ChSonnabend could you have a look? Besides the mechanical change of moving the inference code in a separate .cxx file (this is probably one of the causes why pidTPC takes so much memory to compile), I have also changed the intermediate track representation to be an std::array, which should already somewhat speedup the whole thing. A better approach would be to find a way to have ONNX runtime digest a columnwise representation so that it can bulk process the whole arrow table, rather than one track at the time.

[ChSonnabend]

Hi Giulio,
Thanks a lot for the PR! I think this makes sense to put it in a separate .cxx file. It makes the code much cleaner.
Indeed I also mentioned the idea of running directly on the tables to Nicolo before, but I wasn't sure how to implement it. If you know how to do this, that would be very nice!

[ktf]

@iarsene could you review and eventually approve?

[alibuild]

Error while checking build/O2Physics/o2 for acbbe9d at 2022-07-26 06:59:

## sw/BUILD/O2Physics-latest/log
c++: fatal error: Killed signal terminated program cc1plus
ninja: build stopped: subcommand failed.

Full log here.

[njacazio]

Hi, indeed, thanks a lot this was needed!