[mlgo] Dependency-free training mode logger

This is the next step in dropping the dependency on protobuf.

The simple logger produces an output consisting of lines of json
strings. Tensor values - which should constitute the bulk of the data -
are serialized as raw byte buffers. This allows for light-weight reading
of the values.

The next step is to switch the training logic to the new logging format,
following which the protobuf-based logger will be dropped, together with
the training dependency on protobuf.

Subsequent changes will also stop buffering and stream, instead - the
buffering model is just as a convenient point-in-time.

Differential Revision: https://reviews.llvm.org/D139370