[WebGL2] The outputs don't match the expected values for Concatenation operation

[Christywl]

Test Env:
Chromium Version : nightly build 70.0.3503.0 (revision 3dd7088)
OS : Mac OS
webml-polyfill: commit 777b85f

Expected Result:
Tests should pass.

Actual Result:
The actual outputs don't match the expected values.

• Concatenation Test for 4-D input with axis being 0:
  expected:

[101.0, 102.0, 103.0, 104.0, 105.0, 106.0, 107.0, 108.0,     
 109.0, 110.0, 111.0, 112.0, 113.0, 114.0, 115.0, 116.0,      
 201.0, 202.0, 203.0, 204.0, 205.0, 206.0, 207.0, 208.0,
 209.0, 210.0, 211.0, 212.0, 213.0, 214.0, 215.0, 216.0]

actual:

[0, 0, 0, 0, 0, 0, 0, 0,
 0, 0, 0, 0, 0, 0, 0, 0,
 0, 0, 0, 0, 0, 0, 0, 0,
 0, 0, 0, 0, 0, 0, 0, 0]

• Concatenation Test for 4-D input with axis being 1:
  expected:

[101.0, 102.0, 103.0, 104.0, 105.0, 106.0, 107.0, 108.0,
 201.0, 202.0, 203.0, 204.0, 205.0, 206.0, 207.0, 208.0,
 109.0, 110.0, 111.0, 112.0, 113.0, 114.0, 115.0, 116.0,
 209.0, 210.0, 211.0, 212.0, 213.0, 214.0, 215.0, 216.0]

actual:

[101.0, 102.0, 201.0, 202.0,
 103.0, 104.0, 203.0, 204.0,
 105.0, 106.0, 205.0, 206.0,
 107.0, 108.0, 207.0, 208.0,
 109.0, 110.0, 209.0, 210.0,
 111.0, 112.0, 211.0, 212.0,
 113.0, 114.0, 213.0, 214.0,
 115.0, 116.0, 215.0, 216.0]

• Concatenation Test for 4-D input with axis being 2
  expected:

[101.0, 102.0, 103.0, 104.0,
 201.0, 202.0, 203.0, 204.0,
 105.0, 106.0, 107.0, 108.0,
 205.0, 206.0, 207.0, 208.0,
 109.0, 110.0, 111.0, 112.0,
 209.0, 210.0, 211.0, 212.0,
 113.0, 114.0, 115.0, 116.0,
 213.0, 214.0, 215.0, 216.0]

actual:

[101.0, 102.0, 201.0, 202.0,
 103.0, 104.0, 203.0, 204.0,
 105.0, 106.0, 205.0, 206.0,
 107.0, 108.0, 207.0, 208.0,
 109.0, 110.0, 209.0, 210.0,
 111.0, 112.0, 211.0, 212.0,
 113.0, 114.0, 213.0, 214.0,
 115.0, 116.0, 215.0, 216.0]

How to reproduce:

1. Launch chromium and disable the WebML
2. Run tests: access to https://brucedai.github.io/nt/test/concatenation_012.html?backend=webgl2

[Wenzhao-Xiang]

Looks #56 ,

2. CONCATENATION in WebGL2 version only supports concatenating channel axis(Don't support batch, height and width concatenation).

WebGL2 4D shape is based on 3D shape. Therefore, CONCATENATION in WebGL2 version only supports concatenating channel axis,too.

It will be enhanced if needed.

[GreyZzzzzzXh]

This bug has been fixed by replacing webgl2 with tfjs-core (PR #297).

[Christywl]

All the tests passed on webgl backend on nightly build 6540a81.