Fix MYPY lint: key the torch import exemption on the imported module

The [mypy-.github.scripts.build_report] section never matched: with no
__init__.py in that directory mypy names the module build_report, not the
dotted path, so import-not-found was still reported for torch and
torchvision.

Use ignore_missing_imports on the imported modules instead, which is how mypy
resolves third-party modules that are absent from the lint environment.
Verified with mypy 1.13.0 (the pinned version) against mypy.ini: clean.

Author: atalman

mypy.ini

@@ -15,11 +15,6 @@ disable_error_code = attr-defined, return-value
 [mypy-.github.scripts.run_with_env_secrets]
 disable_error_code = attr-defined, return-value, union-attr
 
-[mypy-.github.scripts.build_report]
-# torch/torchvision are not installed in the lint environment; the script
-# imports them defensively at runtime and degrades when they are absent.
-disable_error_code = import-not-found, import-untyped
-
 [mypy-aws.lambda.whl_metadata_upload_pep658.lambda_function]
 disable_error_code = unused-ignore, import-not-found
 
@@ -75,4 +70,12 @@ disable_error_code = var-annotated, import-untyped
 disable_error_code = index, no-redef, no-untyped-def, import-not-found, no-untyped-call
 
 [mypy-tools.rockset_migration.*]
-disable_error_code = arg-type, attr-defined, return-value, misc, func-returns-value, no-untyped-def, assignment, var-annotated, unused-ignore, truthy-function
+disable_error_code = arg-type, attr-defined, return-value, misc, func-returns-value, no-untyped-def, assignment, var-annotated, unused-ignore, truthy-function
+
+# torch and torchvision are not installed in the lint environment. Scripts that
+# import them do so defensively at runtime and degrade when they are absent.
+[mypy-torch.*]
+ignore_missing_imports = True
+
+[mypy-torchvision.*]
+ignore_missing_imports = True