MLBEDSW-3367 Add mypy to pre-commit

Add mypy to pre-commit and clean up all reported errors.

Signed-off-by: Jonas Ohlsson <jonas.ohlsson@arm.com>
Change-Id: If7dc869f5fecdb0e2db40f14e7d9db21aa33df71
diff --git a/ethosu/vela/high_level_command_stream.py b/ethosu/vela/high_level_command_stream.py
index 7e60221..0009f6c 100644
--- a/ethosu/vela/high_level_command_stream.py
+++ b/ethosu/vela/high_level_command_stream.py
@@ -16,6 +16,7 @@
 # Description:
 # Contains classes that hold commands for the high-level command stream (one command per DMA or NPU stripe).
 from typing import List
+from typing import Optional
 
 import numpy as np
 
@@ -41,8 +42,8 @@
         npu_block_type: NpuBlockType,
         concat_offsets: List[int],
         k_dilated_height: int,
-        split_offset: Shape4D = None,
-        split_shape: Shape4D = None,
+        split_offset: Optional[Shape4D] = None,
+        split_shape: Optional[Shape4D] = None,
         upscaling_factor: int = 1,
     ):
         new_start_coord = list(self.start_coord)