fix: correct typos 'occured' and 'occurence'

Issue facebookresearch#5646: Fix typos in comments
- huffman_coder.py: 'occurence count' -> 'occurrence count'
- dynamic_loss_scaler.py: 'has occured' -> 'has occurred'

This PR has been tested locally.

Author: Jah-yee

fairseq/data/huffman/huffman_coder.py

@@ -173,7 +173,7 @@ def decode(self, bits: bitarray) -> tp.Iterator["HuffmanNode"]:
 
 class HuffmanCodeBuilder:
     """
-    build a dictionary with occurence count and then build the Huffman code for it.
+    build a dictionary with occurrence count and then build the Huffman code for it.
     """
 
     def __init__(self):

fairseq/optim/dynamic_loss_scaler.py

@@ -42,7 +42,7 @@ def _decrease_loss_scale(self):
     def check_overflow(self, grad_norm):
         # detect inf and nan
         if grad_norm == float("inf") or grad_norm != grad_norm:
-            # overflow has occured
+            # overflow has occurred
             prev_scale = self.loss_scale
             iter_since_rescale = self._iter - self._last_rescale_iter