Fixes error with custom scalars, fixes #20579 (#20580)
Summary:
When adding custom scalars like this
```python
from torch.utils.tensorboard import SummaryWriter
with SummaryWriter() as writer:
writer.add_custom_scalars({'Stuff': {
'Losses': ['MultiLine', ['loss/(one|two)']],
'Metrics': ['MultiLine', ['metric/(three|four)']],
}})
```
This error is raised:
```
TypeError: Parameter to MergeFrom() must be instance of same class: expected tensorboard.SummaryMetadata.PluginData got list.
```
Removing the square brackets around `SummaryMetadata.PluginData(plugin_name='custom_scalars')` should be enough to fix it.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20580
Differential Revision: D15469700
Pulled By: orionr
fbshipit-source-id: 7ce58034bc2a74ab149fee6419319db68d8abafe
