[DOC] CoreML frontend tutorial (#2667)

* [DOC] CoreML frontend tutorial

* Update tutorials/frontend/from_coreml.py

Co-Authored-By: kazum <morita.kazutaka@lab.ntt.co.jp>

* Update tutorials/frontend/from_coreml.py

Co-Authored-By: kazum <morita.kazutaka@lab.ntt.co.jp>

* Addressed comments and added the original author

tutorials/frontend/from_coreml.py

+"""
+Compile CoreML Models
+=====================
+**Author**: `Joshua Z. Zhang <https://zhreshold.github.io/>`_, \
+            `Kazutaka Morita <https://github.com/kazum>`_
+
+This article is an introductory tutorial to deploy CoreML models with Relay.
+
+For us to begin with, coremltools module is required to be installed.
+
+A quick solution is to install via pip
+
+.. code-block:: bash
+
+    pip install -U coremltools --user
+
+or please refer to official site
+https://github.com/apple/coremltools
+"""
+import tvm
+import tvm.relay as relay
+import coremltools as cm
+import numpy as np
+from PIL import Image
+
+def download(url, path, overwrite=False):
+    import os
+    if os.path.isfile(path) and not overwrite:
+        print('File {} existed, skip.'.format(path))
+        return
+    print('Downloading from url {} to {}'.format(url, path))
+    try:
+        import urllib.request
+        urllib.request.urlretrieve(url, path)
+    except:
+        import urllib
+        urllib.urlretrieve(url, path)
+
+######################################################################
+# Load pretrained CoreML model
+# ----------------------------
+# We will download and load a pretrained mobilenet classification network
+# provided by apple in this example
+model_url = 'https://docs-assets.developer.apple.com/coreml/models/MobileNet.mlmodel'
+model_file = 'mobilenet.mlmodel'
+download(model_url, model_file)
+# Now you have mobilenet.mlmodel on disk
+mlmodel = cm.models.MLModel(model_file)
+
+######################################################################
+# Load a test image
+# ------------------
+# A single cat dominates the examples!
+img_url = 'https://github.com/dmlc/mxnet.js/blob/master/data/cat.png?raw=true'
+download(img_url, 'cat.png')
+img = Image.open('cat.png').resize((224, 224))
+x = np.transpose(img, (2, 0, 1))[np.newaxis, :]
+
+######################################################################
+# Compile the model on Relay
+# ---------------------------
+# We should be familiar with the process right now.
+target = 'cuda'
+shape_dict = {'image': x.shape}
+
+# Parse CoreML model and convert into Relay computation graph
+func, params = relay.frontend.from_coreml(mlmodel, shape_dict)
+
+with relay.build_config(opt_level=3):
+    graph, lib, params = relay.build(func, target, params=params)
+
+######################################################################
+# Execute on TVM
+# -------------------
+# The process is no different from other example
+from tvm.contrib import graph_runtime
+ctx = tvm.gpu(0)
+dtype = 'float32'
+m = graph_runtime.create(graph, lib, ctx)
+# set inputs
+m.set_input('image', tvm.nd.array(x.astype(dtype)))
+m.set_input(**params)
+# execute
+m.run()
+# get outputs
+tvm_output = m.get_output(0)
+top1 = np.argmax(tvm_output.asnumpy()[0])
+
+#####################################################################
+# Look up synset name
+# -------------------
+# Look up prediction top 1 index in 1000 class synset.
+synset_url = ''.join(['https://gist.githubusercontent.com/zhreshold/',
+                      '4d0b62f3d01426887599d4f7ede23ee5/raw/',
+                      '596b27d23537e5a1b5751d2b0481ef172f58b539/',
+                      'imagenet1000_clsid_to_human.txt'])
+synset_name = 'synset.txt'
+download(synset_url, synset_name)
+with open(synset_name) as f:
+    synset = eval(f.read())
+print('Top-1 id', top1, 'class name', synset[top1])