解释这段代码：import os.path as osp import pandas as pd import torch from sentence_transformers import SentenceTransformer from torch_geometric.data import HeteroData, download_url, extract_zip from torch_geometric.transforms import RandomLinkSplit, ToUndirected url = 'https://files.grouplens.org/datasets/movielens/ml-latest-small.zip' root = osp.join(osp.dirname(osp.realpath(__file__)), '../../data/MovieLens') extract_zip(download_url(url, root), root) movie_path = osp.join(root, 'ml-latest-small', 'movies.csv') rating_path = osp.join(root, 'ml-latest-small', 'ratings.csv') def load_node_csv(path, index_col, encoders=None, **kwargs): df = pd.read_csv(path, index_col=index_col, **kwargs) mapping = {index: i for i, index in enumerate(df.index.unique())} x = None if encoders is not None: xs = [encoder(df[col]) for col, encoder in encoders.items()] x = torch.cat(xs, dim=-1) return x, mapping def load_edge_csv(path, src_index_col, src_mapping, dst_index_col, dst_mapping, encoders=None, **kwargs): df = pd.read_csv(path, **kwargs) src = [src_mapping[index] for index in df[src_index_col]] dst = [dst_mapping[index] for index in df[dst_index_col]] edge_index = torch.tensor([src, dst]) edge_attr = None if encoders is not None: edge_attrs = [encoder(df[col]) for col, encoder in encoders.items()] edge_attr = torch.cat(edge_attrs, dim=-1) return edge_index, edge_attr class SequenceEncoder(object): # The 'SequenceEncoder' encodes raw column strings into embeddings. def __init__(self, model_name='all-MiniLM-L6-v2', device=None): self.device = device self.model = SentenceTransformer(model_name, device=device) @torch.no_grad() def __call__(self, df): x = self.model.encode(df.values, show_progress_bar=True, convert_to_tensor=True, device=self.device) return x.cpu() class GenresEncoder(object)

这段代码是一个Python脚本，主要用于加载MovieLens数据集中的电影和评分数据，并将其转换为图形数据，以便进行图形嵌入。其中，使用了以下库：

- os.path：用于处理文件路径的库。
- pandas：用于处理数据的库。
- torch：用于深度学习的库。
- sentence_transformers：用于生成句子嵌入的库。
- torch_geometric：用于处理图形数据的库。

首先，定义了一个URL和根目录，用于下载和存储MovieLens数据集。然后，定义了两个函数load_node_csv和load_edge_csv，用于加载节点和边的CSV文件，并将它们转换为PyTorch张量。最后，定义了一个名为SequenceEncoder的类，用于将序列数据转换为嵌入向量。