Upgrade MindSpore version to v1.2.0 and make some adaptation

[leonwanghui]

What type of PR is this?

/kind enhancement

What does this PR do / why do we need it:

This PR is proposed to upgrade the dependency of MindSpore to v1.2.0, after that some required modification have been done to make the CI passed.

Besides, this PR has added a new interface load in model module to support load MindIR graph directly to perform model inference operation.

>>> import tinyms as ts
>>> import tinyms.layers as layers
>>> from tinyms.model import Model, load
>>>
>>> net = layers.Conv2d(1, 1, kernel_size=3)
>>> model = Model(net)
>>> input = ts.ones([1, 1, 3, 3])
>>> model.export(input, "net", file_format="MINDIR")
...
>>> net = load("net.mindir")
>>> print(net(input))
[[[[ 0.02548009  0.04010789  0.03120251]
    [ 0.00268656  0.02353744  0.03807815]
    [-0.00896441 -0.00303641  0.01502199]]]]

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewers:

[hannibalhuang]

small nix

[hannibalhuang]

lgtm

[huxiaoman7]

lgtm