Created by: chenwhql

In order to get simpler and clearer debug strings, add _to_readable_code method to Program, Block, Operator, Variable, and change method __str__ to call to_readable_code method by default.

• There is almost no information loss in this conversion, but the information density is increased

• why add new API instead of changing old method to_string?

  • to_string interface has been existed for a long time, not sure if changing it will cause compatibility problems for some users
  • keep to_string interface unchanged, users still can easily get the original ptotobuf string through to_string, or print(Program.to_string())

For a same program:

• The new debug string example:

{ // block 0
    var X : fluid.VarType.LOD_TENSOR.shape(-1, 23, 48).astype(VarType.FP32)
    persist trainable param W : fluid.VarType.LOD_TENSOR.shape(23, 48).astype(VarType.FP32)

    {Out=['X']} = abs(inputs={X=['X']}, op_device = , op_namescope = /, op_role = 0, op_role_var = [], use_cudnn = False, use_mkldnn = False)
}

• The old debug string example:

blocks {
  idx: 0
  parent_idx: -1
  vars {
    name: "W"
    type {
      type: LOD_TENSOR
      lod_tensor {
        tensor {
          data_type: FP32
          dims: 23
          dims: 48
        }
      }
    }
    persistable: true
  }
  vars {
    name: "X"
    type {
      type: LOD_TENSOR
      lod_tensor {
        tensor {
          data_type: FP32
          dims: -1
          dims: 23
          dims: 48
        }
        lod_level: 0
      }
    }
  }
  ops {
    inputs {
      parameter: "X"
      arguments: "X"
    }
    outputs {
      parameter: "Out"
      arguments: "X"
    }
    type: "abs"
    attrs {
      name: "op_device"
      type: STRING
      s: ""
    }
    attrs {
      name: "use_cudnn"
      type: BOOLEAN
      b: false
    }
    attrs {
      name: "op_role"
      type: INT
      i: 0
    }
    attrs {
      name: "op_role_var"
      type: STRINGS
    }
    attrs {
      name: "op_callstack"
      type: STRINGS
      strings: "  File \"/usr/local/lib/python3.5/dist-packages/paddle/fluid/framework.py\", line 2730, in append_op\n    attrs=kwargs.get(\"attrs\", None))\n"
      strings: "  File \"../python/paddle/fluid/tests/unittests/test_program_code.py\", line 90, in setUp\n    type=\"abs\", inputs={\"X\": [self.var]}, outputs={\"Out\": [self.var]})\n"
      strings: "  File \"/usr/lib/python3.5/unittest/case.py\", line 596, in run\n    self.setUp()\n"
      strings: "  File \"/usr/lib/python3.5/unittest/case.py\", line 648, in __call__\n    return self.run(*args, **kwds)\n"
      strings: "  File \"/usr/lib/python3.5/unittest/suite.py\", line 122, in run\n    test(result)\n"
      strings: "  File \"/usr/lib/python3.5/unittest/suite.py\", line 84, in __call__\n    return self.run(*args, **kwds)\n"
      strings: "  File \"/usr/lib/python3.5/unittest/suite.py\", line 122, in run\n    test(result)\n"
      strings: "  File \"/usr/lib/python3.5/unittest/suite.py\", line 84, in __call__\n    return self.run(*args, **kwds)\n"
      strings: "  File \"/usr/lib/python3.5/unittest/runner.py\", line 176, in run\n    test(result)\n"
      strings: "  File \"/usr/lib/python3.5/unittest/main.py\", line 255, in runTests\n    self.result = testRunner.run(self.test)\n"
      strings: "  File \"/usr/lib/python3.5/unittest/main.py\", line 94, in __init__\n    self.runTests()\n"
      strings: "  File \"../python/paddle/fluid/tests/unittests/test_program_code.py\", line 117, in <module>\n    unittest.main()\n"
    }
    attrs {
      name: "op_namescope"
      type: STRING
      s: "/"
    }
    attrs {
      name: "use_mkldnn"
      type: BOOLEAN
      b: false
    }
  }
}
version {
  version: 0
}