怎么解决无法从sklearn.datasets中导入fetch_mldata。

`fetch_mldata`已经被弃用，所以无法从`sklearn.datasets`中导入。您可以尝试以下替代方案：

1. 使用`fetch_openml`代替`fetch_mldata`，例如：

   from sklearn.datasets import fetch_openml
   mnist = fetch_openml('mnist_784')

2. 从外部下载数据集并手动加载数据集，例如：

   import numpy as np
   import gzip
   
   def load_mnist(path, kind='train'):
       """Load MNIST data from `path`"""
       labels_path = os.path.join(path, '%s-labels-idx1-ubyte.gz' % kind)
       images_path = os.path.join(path, '%s-images-idx3-ubyte.gz' % kind)
   
       with gzip.open(labels_path, 'rb') as lbpath:
           labels = np.frombuffer(lbpath.read(), dtype=np.uint8, offset=8)
   
       with gzip.open(images_path, 'rb') as imgpath:
           images = np.frombuffer(imgpath.read(), dtype=np.uint8, offset=16).reshape(len(labels), 784)
   
       return images, labels
   
   X_train, y_train = load_mnist('path/to/mnist', kind='train')
   X_test, y_test = load_mnist('path/to/mnist', kind='t10k')