Update plot_train.py

[zhubonan]

Fix PDF file names of the mace_plot_train command. . Previous, the names of the output files are tuples:

('<my_filename>',).pdf

is this due to passing a list of grouped keys to the groupby, which results in a tuple of grouped keys being returned.
Since we are only grouping a single key ('name'), passing a string should be sufficient.

[ilyes319]

Thanks @zhubonan !