Add 'validation_split_percentage' and 'evaluation_strategy' parameters for Trainers

[smirn0v]

Is your feature request related to a problem? Please describe. Currently 'validation_split_percentage' is set to 0 and changing it to any other value will have no effect cause 'evaluation_strategy' not set at all and by default 'no'

Describe the solution you'd like Please add 'validation_split_percentage' and 'evaluation_strategy' parameters to control training process.

[research4pan]

Thanks for your contribution to LMFlow! We will schedule this feature in our next version 😄