Updated SparseML callback for latest PyTorch Lightning (#822)

* Updated callback
* Fix for `max_steps` (defaults to `-1`),
* Fix for deprecation of `num_gpus`, `num_processes` and `tpu_cores`.
* Avoid breaking changes for `max_steps=None`
* Added comments for clarity and future maintenance

pl_bolts/callbacks/sparseml.py

@@ -66,15 +66,22 @@ def _num_training_steps_per_epoch(self, trainer: Trainer) -> int:
         else:
             dataset_size = len(trainer.datamodule.train_dataloader())
 
-        num_devices = max(1, trainer.num_gpus, trainer.num_processes)
-        if trainer.tpu_cores:
-            num_devices = max(num_devices, trainer.tpu_cores)
+        if hasattr(trainer, 'num_devices'):
+            # New behavior in Lightning
+            num_devices = max(1, trainer.num_devices)
+        else:
+            # Old behavior deprecated in v1.6
+            num_devices = max(1, trainer.num_gpus, trainer.num_processes)
+            if trainer.tpu_cores:
+                num_devices = max(num_devices, trainer.tpu_cores)
 
         effective_batch_size = trainer.accumulate_grad_batches * num_devices
         max_estimated_steps = dataset_size // effective_batch_size
 
-        if trainer.max_steps and trainer.max_steps < max_estimated_steps:
-            return trainer.max_steps
+        # To avoid breaking changes, max_steps is set to -1 if it is not defined
+        max_steps = -1 if not trainer.max_steps else trainer.max_steps
+        if max_steps != -1 and max_steps < max_estimated_steps:
+            return max_steps
         return max_estimated_steps
 
     @staticmethod