Fixed warning message in BLEURT default initialization

[zafercavdar]

Jury constructor accepts metrics as a string, an object from Metric class or list of metric configurations inside a dict. In addition, BLEURT metric checks for config_namekey instead of checkpoint key. Thus, this warning message misleads if default model is not used.

Here is an example of incorrect initialization and warning message:

checkpoint is ignored:

[devrimcavusoglu]

@zafercavdar Thank you for the contribution.