Fix nodes import tests

Context

This MR is dedicated to enhance some operators imports mentioned in issue #46.
The operators that will be enhanced are the following:

• gemm: * missing alpha and beta attributes.
  * missing transA and transB attributes.
  * cannot support bias of shape [1, outChannels].
• Sum: can no longer use Add operator because the latter can no longer support more than 2 inputs.

Modified files

• gemm.py, add the attrs alpha, beta, transA and transB and support bias of shape [1, outChannels];
• sum.py, add sumimport/export;

TODO

• Add alpha and beta attr to FC operator
• Add transA and transB attr to FC oprator
• Support bias of shape [1, outChannels] in FC oprator
• Add Sum operator