[PIR] No.1 Migrate paddle.mean into pir

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Description
[PIR]Migrate layer_norm into pir
总计 20 个单测，打开13个单测，7个单测继承自unittest.TestCase没有覆盖到

Pcard-67164

def mean(x, axis=None, keepdim=False, name=None):
    if in_dygraph_mode():
        # 动态图分支代码
        return _C_ops.mean(x, axis, keepdim)
    else:
        # 静态图分支代码，省略

def mean(x, axis=None, keepdim=False, name=None):
    if in_dynamic_or_pir_mode():
        # 动态图 / 新IR分支代码
        return _C_ops.mean(x, axis, keepdim)   # <----- 动静统一
    else:
        # 静态图分支代码，省略

def concat(x, axis=0, name=None):
    input = x
    if in_dynamic_mode():
        # 动态图代码分支
        if isinstance(axis, Variable):
            axis = axis.item(0)
        if not isinstance(input, Variable):
            input = [t for t in input if t.shape.count(0) == 0]
        return _C_ops.concat(input, axis)
    else:
        # 静态图代码分支

def concat(x, axis=0, name=None):
    input = x
    if in_dynamic_mode():
        # 动态图代码分支
        if isinstance(axis, Variable):
            axis = axis.item(0)
        if not isinstance(input, Variable):
            input = [t for t in input if t.shape.count(0) == 0]
        return _C_ops.concat(input, axis)
    elif in_pir_mode():
        # 新IR代码分支
        if not isinstance(input, paddle.ir.Value):
            input = [t for t in input if t.shape.count(0) == 0]
        return _C_ops.concat(input, axis)
    else:
        # 静态图代码分支

class TestMeanOp(OpTest):
    # 其他代码省略
    def test_check_output(self):
        self.check_output(check_new_ir=True)

    def test_checkout_grad(self):
        self.check_grad(['X'], 'Out', check_new_ir=True)