[FEATURE] Logging frequency for TensorBoard
Is your feature request related to a problem? Please describe.
TensorBoard is a great tool for visualization but the size of event files rapidly grows bigger when we log images.
To prevent this problem, we need to set logging frequencies.
As the best of  my knowledge, in pytorch-lightning, we can manually set them like:
class CoolModule(pl.LightningModule):
    def __init__(self, args):
        self.log_feq = args.log_freq

        self.model = ...

    def training_step(self, data_batch, batch_nb):
        input = data_batch['input']
        output = self.forward(input)

        if self.global_step % self.log_freq == 0:
            self.experiment.add_image('output_image', output, self.global_step)
This is an easy way, however, I think it is clearer to control frequencies by Trainer.
Describe the solution you'd like
Add an option for controlling the frequencies to Trainer.
trainer = Trainer(model, tb_log_freq=foo)
To enable this functionality, training_step should return image tensors like:
def training_step(self, data_batch, batch_nb):
    input = data_batch['input']
    output = self.forward(input)
    loss = ...
    return {'loss', loss, 'prog': {'loss': loss}, 'image': {'output': output}}