standard weekly patch release

Detail changes

Added

• Added PyTorch 1.7 Stable support (#3821)
• Added timeout for tpu_device_exists to ensure process does not hang indefinitely (#4340)

Changed

• W&B log in sync with Trainer step (#4405)
• Hook on_after_backward is called only when optimizer_step is being called (#4439)
• Moved track_and_norm_grad into training loop and called only when optimizer_step is being called (#4439)
• Changed type checker with explicit cast of ref_model object (#4457)

Deprecated

• Deprecated passing ModelCheckpoint instance to checkpoint_callback Trainer argument (#4336)

Fixed

• Disable saving checkpoints if not trained (#4372)
• Fixed error using auto_select_gpus=True with gpus=-1 (#4209)
• Disabled training when limit_train_batches=0 (#4371)
• Fixed that metrics do not store computational graph for all seen data (#4313)
• Fixed AMP unscale for on_after_backward (#4439)
• Fixed TorchScript export when module includes Metrics (#4428)
• Fixed CSV logger warning (#4419)
• Fixed skip DDP parameter sync (#4301)

Contributors

@ananthsub, @awaelchli, @borisdayma, @carmocca, @justusschock, @lezwon, @rohitgr7, @SeanNaren, @SkafteNicki, @ssaru, @tchaton, @ydcjeff

If we forgot someone due to not matching commit email with GitHub account, let us know :]