Add missing submission.py for vectoradd_py problems

The popcorn-cli setup command expects every problem to have a
submission.py, but pmpp/vectoradd_py and pmpp_v2/vectoradd_py only had
solutions under correct/. Add simple PyTorch baselines so the CLI can
bootstrap these problems.

Author: Mark Saroufim

problems/pmpp/vectoradd_py/submission.py

@@ -0,0 +1,6 @@
+from task import input_t, output_t
+
+
+def custom_kernel(data: input_t) -> output_t:
+    A, B = data
+    return A + B

problems/pmpp_v2/vectoradd_py/submission.py

@@ -0,0 +1,7 @@
+from task import input_t, output_t
+
+
+def custom_kernel(data: input_t) -> output_t:
+    A, B, output = data
+    output[...] = A + B
+    return output