Avoid runtime exception when file doesn't exist

[sf-wind]

When running auto-tuning, autoTVM tries to read from history run logs. However, if the log file doesn't exist, and cannot download from the web, the run would fail. That is because load_from_file(records)
returns a generator, which is guaranteed to be not None. So it would fail in line 279 when iterating the records.

I believe we need to return in case the records does not exist.

[eqy]

I think there is a check for that the tophub package has been downloaded:
https://github.com/dmlc/tvm/blob/a9bd5593a35ce4717f00cc71c2377f745284df5a/python/tvm/autotvm/tophub.py#L172
Can you give an example of a test case where the current implementation fails?

[sf-wind]

The trace is:

File "/data/users/feisun/tvm/venv/lib/python2.7/site-packages/nnvm-0.8.0-py2.7.egg/nnvm/compiler/build_module.py", line 245, in build [0/1821]
tophub_context = autotvm.tophub.context(target)
File "/data/users/feisun/tvm/venv/lib/python2.7/site-packages/tvm-0.5.dev0-py2.7-linux-x86_64.egg/tvm/autotvm/tophub.py", line 83, in context
best_context.load(os.path.join(AUTOTVM_TOPHUB_ROOT_PATH, filename))
File "/data/users/feisun/tvm/venv/lib/python2.7/site-packages/tvm-0.5.dev0-py2.7-linux-x86_64.egg/tvm/autotvm/task/dispatcher.py", line 282, in load
for inp, res in records:
File "/data/users/feisun/tvm/venv/lib/python2.7/site-packages/tvm-0.5.dev0-py2.7-linux-x86_64.egg/tvm/autotvm/record.py", line 166, in load_from_file
for row in open(filename):
IOError: [Errno 2] No such file or directory: '/home/feisun/.tvm/tophub/llvm_v0.03.log'

[eqy]

That is interesting; did you see the logging.warning("Failed to download tophub package for %s: %s", backend, e) warning? check_backend(name) should be called before this.

[sf-wind]

Yes, I saw:

INFO:autotvm:Download pre-tuned parameters package llvm_v0.03.log
WARNING:root:Failed to download tophub package for llvm: <urlopen error [Errno 101] Network is unreachable>

[merrymercy]

We'd better move the error handling to a place within TopHub. Because we want to hide this error for TopHub but we don't want to hide this error for other cases. "cannot find file" is a fatal error that worth an exception.

We can move the error handling to python/tvm/autotvm/tophub.py
in check_backend, return a True if it successes(after L108, L115), returns a False if it fails (after L117)
Then in L80, handle this return value.

[merrymercy]

Thanks @sf-wind @eqy. It is merged.