Make lod_append support variable lod. (#18908)

test=develop

paddle/fluid/API.spec

@@ -164,7 +164,7 @@ paddle.fluid.layers.reshape (ArgSpec(args=['x', 'shape', 'actual_shape', 'act',
 paddle.fluid.layers.squeeze (ArgSpec(args=['input', 'axes', 'name'], varargs=None, keywords=None, defaults=(None,)), ('document', 'ebbac07662a6e22e8e299ced880c7775'))
 paddle.fluid.layers.unsqueeze (ArgSpec(args=['input', 'axes', 'name'], varargs=None, keywords=None, defaults=(None,)), ('document', 'b9bd3129d36a70e7c4385df51ff71c62'))
 paddle.fluid.layers.lod_reset (ArgSpec(args=['x', 'y', 'target_lod'], varargs=None, keywords=None, defaults=(None, None)), ('document', '74498d37dd622ac472cb36887fce09ea'))
-paddle.fluid.layers.lod_append (ArgSpec(args=['x', 'level'], varargs=None, keywords=None, defaults=None), ('document', '527cc61c20a8fa83008115440419f92b'))
+paddle.fluid.layers.lod_append (ArgSpec(args=['x', 'level'], varargs=None, keywords=None, defaults=None), ('document', '37663c7c179e920838a250ea0e28d909'))
 paddle.fluid.layers.lrn (ArgSpec(args=['input', 'n', 'k', 'alpha', 'beta', 'name'], varargs=None, keywords=None, defaults=(5, 1.0, 0.0001, 0.75, None)), ('document', '73d297256da8954617996958d26ee93d'))
 paddle.fluid.layers.pad (ArgSpec(args=['x', 'paddings', 'pad_value', 'name'], varargs=None, keywords=None, defaults=(0.0, None)), ('document', '2f189f8ef61f1c23779e1593b78755c0'))
 paddle.fluid.layers.pad_constant_like (ArgSpec(args=['x', 'y', 'pad_value', 'name'], varargs=None, keywords=None, defaults=(0.0, None)), ('document', '95aa1972983f30fe9b5a3713e523e20f'))

python/paddle/fluid/layers/nn.py

@@ -7099,7 +7099,7 @@ def lod_append(x, level):
 
     Args:
         x (Variable): Input variable which could be a tensor or LoDTensor.
-        level (list|tuple): The LoD level to be appended into LoD of x.
+        level (list|tuple|Variable): The LoD level to be appended into LoD of x.
 
     Returns:
         Variable: Output variable with new LoD level.
@@ -7117,16 +7117,21 @@ def lod_append(x, level):
     from collections import Iterable
     if x is None:
         raise ValueError("Input(x) can't be None.")
-    if not isinstance(level, Iterable):
-        raise ValueError("Input(level) must be list or tuple.")
+    if (not isinstance(level, Iterable)) and (not isinstance(level, Variable)):
+        raise ValueError("Input(level) must be list, tuple or Variable.")
+
     helper = LayerHelper("lod_append", **locals())
     out = helper.create_variable_for_type_inference(dtype=x.dtype)
+
+    inputs = {'X': x}
+    attrs = {'append': True}
+
+    if isinstance(level, Variable):
+        inputs['Y'] = level
+    else:
+        attrs['target_lod'] = level
     helper.append_op(
-        type="lod_reset",
-        inputs={'X': x},
-        attrs={'target_lod': level,
-               'append': True},
-        outputs={'Out': out})
+        type="lod_reset", inputs=inputs, attrs=attrs, outputs={'Out': out})
     return out