update lecun_normal in initializer.py and fix __all__ and API doc (#1187)

Author: HydrogenSulfate

docs/zh/api/utils/initializer.md

@@ -16,5 +16,7 @@
         - kaiming_normal_
         - linear_init_
         - conv_init_
+        - glorot_normal_
+        - lecun_normal_
       show_root_heading: True
       heading_level: 3

ppsci/utils/initializer.py

@@ -46,6 +46,8 @@
     "kaiming_normal_",
     "linear_init_",
     "conv_init_",
+    "glorot_normal_",
+    "lecun_normal_",
 ]
 
 
@@ -496,3 +498,32 @@ def glorot_normal_(tensor: paddle.Tensor) -> paddle.Tensor:
     trunc_normal_(tensor)
     tensor.set_value(tensor * stddev)
     return tensor
+
+
+def lecun_normal_(tensor: paddle.Tensor) -> paddle.Tensor:
+    """Modify tensor inplace using jax-style lecun_normal.
+
+    References:
+        https://github.com/jax-ml/jax/blob/main/jax/_src/nn/initializers.py#L480-L513
+
+    Args:
+        tensor (paddle.Tensor): Paddle Tensor/Parameter.
+
+    Returns:
+        paddle.Tensor: Initialized tensor.
+
+    Examples:
+        >>> import paddle
+        >>> import ppsci
+        >>> param = paddle.empty((128, 256), "float32")
+        >>> param = ppsci.utils.initializer.lecun_normal_(param)
+    """
+    assert (
+        tensor.ndim == 2
+    ), f"lecun_normal_ only support 2D tensor now, but got ndim={tensor.ndim}"
+    fin, _ = tensor.shape
+    var = 1.0 / fin
+    stddev = math.sqrt(var) / 0.87962566103423978
+    trunc_normal_(tensor)
+    tensor.set_value(tensor * stddev)
+    return tensor