Fix destructor for delayed mmap dataset case (#6703)

Signed-off-by: Mikołaj Błaż <[email protected]>
Co-authored-by: Eric Harper <[email protected]>

nemo/collections/nlp/data/language_modeling/megatron/indexed_dataset.py

@@ -513,6 +513,8 @@ def _do_init(self, path, skip_warmup=True, delay_data_mmap=False):
             self._create_data_mmap(skip_warmup)
         else:
             logging.info("    skip creating data numpy buffer of mmap...")
+            self._bin_buffer_mmap = None
+            self._bin_buffer = None
 
     def _create_data_mmap(self, skip_warmup):
         if not skip_warmup:
@@ -524,7 +526,8 @@ def _create_data_mmap(self, skip_warmup):
         self._bin_buffer = memoryview(self._bin_buffer_mmap)
 
     def __del__(self):
-        self._bin_buffer_mmap._mmap.close()
+        if self._bin_buffer_mmap is not None:
+            self._bin_buffer_mmap._mmap.close()
         del self._bin_buffer_mmap
         del self._index