Adddocumentation for preemption suppoort (#6403)

Signed-off-by: Abhishree <[email protected]>

docs/source/core/exp_manager.rst

@@ -173,9 +173,20 @@ and stability. To use EMA, simply set the following via YAML or :class:`~nemo.ut
             every_n_steps: 1  # How often to update EMA weights
             validate_original_weights: False  # Whether to use original weights for validation calculation or EMA weights
 
+Support for Preemption
+----------------------
+
+.. _exp_manager_preemption_support-label:
+
+NeMo adds support for a callback upon preemption while running the models on clusters. The callback takes care of saving the current state of training via the ``.ckpt``
+file followed by a graceful exit from the run. The checkpoint saved upon preemption has the ``*last.ckpt`` suffix and replaces the previously saved last checkpoints.
+This feature is useful to increase utilization on clusters.
+The ``PreemptionCallback`` is enabled by default. To disable it simply add ``create_preemption_callback: False`` under exp_manager in the config YAML file. 
+
 
 .. _nemo_multirun-label:
 
+
 Hydra Multi-Run with NeMo
 -------------------------