Fix MultilineChartContent not defined

[ldeso]

Calling the function log_custom_scalar results in an error:

ERROR: UndefVarError: `MultilineChartContent` not defined

This fixes the error by importing the missing undefined classes.

Those classes are defined in src/protojl/tensorboard/plugins/custom_scalar/tensorboard/layout_pb.jl.

This is similar to what was done in the file for the audio logger src/Loggers/LogAudio.jl.