Add CI scripts for Custom Build (pytorch#32316)

Summary:
Pull Request resolved: pytorch#32316

### Summary

Since the Custom Build has been released in 1.4.0, it's time setup CI. To do that, we need

1.  Add a python script to generate the yaml file
2. Add new build scripts to circle CI (arm64 only).

### Test Plan

- Don't break the current iOS CIs

Test Plan: Imported from OSS

Differential Revision: D19437362

Pulled By: xta0

fbshipit-source-id: 395e27a582c43663af88d11b1ef974a4687e672c

ios/TestApp/custom_build/custom_build.py

@@ -0,0 +1,11 @@
+import torch
+import torchvision
+import yaml
+
+model = torchvision.models.mobilenet_v2(pretrained=True)
+model.eval()
+example = torch.rand(1, 3, 224, 224)
+traced_script_module = torch.jit.trace(model, example)
+ops = torch.jit.export_opnames(traced_script_module)
+with open('mobilenetv2.yaml', 'w') as output:
+    yaml.dump(ops, output)

ios/TestApp/custom_build/mobilenetv2.yaml

@@ -0,0 +1,10 @@
+- aten::_convolution
+- aten::add.Tensor
+- aten::addmm
+- aten::batch_norm
+- aten::dropout
+- aten::hardtanh_
+- aten::mean.dim
+- aten::t
+- prim::Constant
+- prim::ListConstruct