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/operation_util.py b/ethosu/vela/operation_util.py
index 29caf6d..36a8e59 100644
--- a/ethosu/vela/operation_util.py
+++ b/ethosu/vela/operation_util.py
@@ -234,7 +234,7 @@
     op_type: Op,
     name: str,
     ifm: Tensor,
-    ifm2: Tensor,
+    ifm2: Optional[Tensor],
     quantization: QuantizationParameters,
     activation: Optional[ActivationFunction] = None,
     dtype: Optional[DataType] = None,