graph_analysis: nicer json parsed graph structure

[colah]

Nicer layout parse structure for @shancarter:

"layout": {
    "type": "Sequence",
    "children": [
      {
        "type": "Node",
        "name": "input:0"
      },
      {
        "type": "Node",
        "name": "conv2d0:0"
      },
      {
        "type": "Node",
        "name": "maxpool0:0"
      },
      {
        "type": "Node",
        "name": "conv2d1:0"
      },
      {
        "type": "Node",
        "name": "conv2d2:0"
      },
      {
        "type": "Node",
        "name": "maxpool1:0"
      },
      {
        "type": "Branch",
        "children": [
          {
            "type": "Node",
            "name": "mixed3a_1x1:0"
          },
          {
            "type": "Sequence",
            "children": [
              {
                "type": "Node",
                "name": "mixed3a_3x3_bottleneck:0"
              },
              {
                "type": "Node",
                "name": "mixed3a_3x3:0"
              }
            ]
          },
...

[colah]

This change probably breaks some things.

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[ludwigschubert]

Sorry for the mess, everyone! I tried to rebase this on master, but ended up creating a huge merge commit. I've cherry picked @colah 's changes instead, and they're merged on master.