[MPS] ADD is not supported for MPS

[Christywl]

ADD operation is not supported for MPS: https://github.com/intel/webml-polyfill/blob/master/docs/supported_ops.md

Op Type	MPS
ADD	no
MUL	no

[huningxin]

Split the MUL into #117

[huningxin]

After review the ADD related test cases: https://github.com/intel/webml-polyfill/blob/master/test/operations/add.js https://github.com/intel/webml-polyfill/blob/master/test/cts/test/V1_0/add.js

and the examples: https://github.com/intel/webml-polyfill/tree/master/examples/simple (also requires MUL op support) https://github.com/intel/webml-polyfill/tree/master/examples/mobilenet (by using the MobileNet v2 model)

The TODOs include:

• [x] need to support tensor with dimension 1 (tests)
• [x] need to support uploading multiple model inputs to MPS images (simple example)
• [x] need to support uploading constants of ADD op to MPS images (tests)
• [x] create MPSCNNAdd instance when compile the model (all)
• [x] link the inputs and output of op when execute the model (all)
• [ ] support fused RELU (no test case yet)

This would also setup a base to add MUL which only requires to change step 4 to MPSCNNMultiply

[huningxin]

Fixed. @BruceDai , please verify. Thanks!

[Christywl]

Verified on nightly build b7ce158, the tests passed. https://brucedai.github.io/nt/test/add.html?backend=mps