add sanity check to input shape type

[kazum]

This PR adds a sanity check to the input shape type of nnvm.compiler.build.
Without this patch, the error message is confusing, especially to beginners.

>>> nnvm.compiler.build(compute_graph, target="cuda", shape={"x": None})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/kazutaka/git/nnvm/python/nnvm/compiler/build_module.py", line 257, in build
    ishape, _ = graph_util.infer_shape(graph, **shape)
  File "/Users/kazutaka/git/nnvm/python/nnvm/compiler/graph_util.py", line 30, in infer_shape
    graph = graph_attr.set_shape_inputs(graph, shape)
  File "/Users/kazutaka/git/nnvm/python/nnvm/compiler/graph_attr.py", line 25, in set_shape_inputs
    g._set_json_attr("shape_inputs", list_shape, 'list_shape')
  File "/Users/kazutaka/git/nnvm/python/nnvm/graph.py", line 165, in _set_json_attr
    self.handle, c_str(key), c_str(json_value)))
  File "/Users/kazutaka/git/nnvm/python/nnvm/_base.py", line 75, in check_call
    raise NNVMError(py_str(_LIB.NNGetLastError()))
nnvm._base.NNVMError: [10:10:36] /Users/kazutaka/git/nnvm/dmlc-core/include/dmlc/./json.h:738: Check failed: ch == '[' (110 vs. [) Error at Line 0, around ^`ull, []]]`, Expect '{' but get 'n'

Stack trace returned 10 entries:
[bt] (0) 0   libnnvm_compiler.dylib              0x0000000112a61edb dmlc::StackTrace() + 299
[bt] (1) 1   libnnvm_compiler.dylib              0x0000000112a61c6f dmlc::LogMessageFatal::~LogMessageFatal() + 47
[bt] (2) 2   libnnvm_compiler.dylib              0x0000000112b981a9 dmlc::JSONReader::BeginArray() + 505
[bt] (3) 3   libnnvm_compiler.dylib              0x0000000112bcefec nnvm::Tuple<long long>::Load(dmlc::JSONReader*) + 44
[bt] (4) 4   libnnvm_compiler.dylib              0x0000000112bceaa1 dmlc::json::ArrayHandler<std::__1::vector<nnvm::TShape, std::__1::allocator<nnvm::TShape> > >::Read(dmlc::JSONReader*, std::__1::vector<nnvm::TShape, std::__1::allocator<nnvm::TShape> >*) + 129
[bt] (5) 5   libnnvm_compiler.dylib              0x0000000112bce844 void dmlc::json::AnyJSONManager::ReadAny<std::__1::vector<nnvm::TShape, std::__1::allocator<nnvm::TShape> > >(dmlc::JSONReader*, dmlc::any*) + 52
[bt] (6) 6   libnnvm_compiler.dylib              0x0000000112b97e70 dmlc::json::Handler<dmlc::any>::Read(dmlc::JSONReader*, dmlc::any*) + 848
[bt] (7) 7   libnnvm_compiler.dylib              0x0000000112b96753 NNGraphSetJSONAttr + 483
[bt] (8) 8   _ctypes.so                          0x000000010d15c837 ffi_call_unix64 + 79
[bt] (9) 9   ???                                 0x00007fff52e1e570 0x0 + 140734583924080