add IterableDataset.shard (and rename n_shards -> num_shards)

docs/source/about_mapstyle_vs_iterable.mdx

@@ -139,12 +139,12 @@ But using a shuffle buffer is not enough to provide a satisfactory shuffling for
 ```python
 # Stream from the internet
 my_iterable_dataset = load_dataset("deepmind/code_contests", split="train", streaming=True)
-my_iterable_dataset.n_shards  # 39
+my_iterable_dataset.num_shards  # 39
 
 # Stream from local files
 data_files = {"train": [f"path/to/data_{i}.csv" for i in range(1024)]}
 my_iterable_dataset = load_dataset("csv", data_files=data_files, split="train", streaming=True)
-my_iterable_dataset.n_shards  # 1024
+my_iterable_dataset.num_shards  # 1024
 
 # From a generator function
 def my_generator(n, sources):
@@ -154,7 +154,7 @@ def my_generator(n, sources):
 
 gen_kwargs = {"n": 10, "sources": [f"path/to/data_{i}" for i in range(1024)]}
 my_iterable_dataset = IterableDataset.from_generator(my_generator, gen_kwargs=gen_kwargs)
-my_iterable_dataset.n_shards  # 1024
+my_iterable_dataset.num_shards  # 1024
 ```
 
 ## Speed differences
@@ -242,5 +242,5 @@ my_iterable_dataset = my_dataset.to_iterable_dataset()
 If you want to shuffle your dataset or [use it with a PyTorch DataLoader](./use_with_pytorch#stream-data), we recommend generating a sharded [`IterableDataset`]:
 ```python
 my_iterable_dataset = my_dataset.to_iterable_dataset(num_shards=1024)
-my_iterable_dataset.n_shards  # 1024
+my_iterable_dataset.num_shards  # 1024
 ```

docs/source/use_with_pytorch.mdx

@@ -216,7 +216,7 @@ If the dataset is split in several shards (i.e. if the dataset consists of multi
 
 ```py
 >>> my_iterable_dataset = load_dataset("deepmind/code_contests", streaming=True, split="train")
->>> my_iterable_dataset.n_shards
+>>> my_iterable_dataset.num_shards
 39
 >>> dataloader = DataLoader(my_iterable_dataset, batch_size=32, num_workers=4)
 ```
@@ -259,7 +259,7 @@ Each node is assigned a chunk of data, e.g. rank 0 is given the first chunk of t
 
 For iterable datasets:
 
-If the dataset has a number of shards that is a factor of `world_size` (i.e. if `dataset.n_shards % world_size == 0`),
+If the dataset has a number of shards that is a factor of `world_size` (i.e. if `dataset.num_shards % world_size == 0`),
 then the shards are evenly assigned across the nodes, which is the most optimized.
 Otherwise, each node keeps 1 example out of `world_size`, skipping the other examples.
 

src/datasets/arrow_dataset.py

@@ -4630,40 +4630,40 @@ def shard(
         self,
         num_shards: int,
         index: int,
-        contiguous: bool = False,
+        contiguous: bool = True,
         keep_in_memory: bool = False,
         indices_cache_file_name: Optional[str] = None,
         writer_batch_size: Optional[int] = 1000,
     ) -> "Dataset":
         """Return the `index`-nth shard from dataset split into `num_shards` pieces.
 
-        This shards deterministically. `dset.shard(n, i)` will contain all elements of dset whose
-        index mod `n = i`.
+        This shards deterministically. `dataset.shard(n, i)` splits the dataset into contiguous chunks,
+        so it can be easily concatenated back together after processing. If `len(dataset) % n == l`, then the
+        first `l` dataset each have length `(len(dataset) // n) + 1`, and the remaining dataset have length `(len(dataset) // n)`.
+        `datasets.concatenate([dset.shard(n, i) for i in range(n)])` returns a dataset with the same order as the original.
 
-        `dset.shard(n, i, contiguous=True)` will instead split dset into contiguous chunks,
-        so it can be easily concatenated back together after processing. If `n % i == l`, then the
-        first `l` shards will have length `(n // i) + 1`, and the remaining shards will have length `(n // i)`.
-        `datasets.concatenate([dset.shard(n, i, contiguous=True) for i in range(n)])` will return
-        a dataset with the same order as the original.
+        Note: n should be less or equal to the number of elements in the dataset `len(dataset)`.
+
+        On the other hand, `dataset.shard(n, i, contiguous=False)` contains all elements of the dataset whose index mod `n = i`.
 
         Be sure to shard before using any randomizing operator (such as `shuffle`).
         It is best if the shard operator is used early in the dataset pipeline.
 
-
         Args:
             num_shards (`int`):
                 How many shards to split the dataset into.
             index (`int`):
                 Which shard to select and return.
-            contiguous: (`bool`, defaults to `False`):
+            contiguous: (`bool`, defaults to `True`):
                 Whether to select contiguous blocks of indices for shards.
             keep_in_memory (`bool`, defaults to `False`):
                 Keep the dataset in memory instead of writing it to a cache file.
             indices_cache_file_name (`str`, *optional*):
                 Provide the name of a path for the cache file. It is used to store the
                 indices of each shard instead of the automatically generated cache file name.
             writer_batch_size (`int`, defaults to `1000`):
-                Number of rows per write operation for the cache file writer.
+                This only concerns the indices mapping.
+                Number of indices per write operation for the cache file writer.
                 This value is a good trade-off between memory usage during the processing, and processing speed.
                 Higher value makes the processing do fewer lookups, lower value consume less temporary memory while running `map`.
 

src/datasets/distributed.py

@@ -18,7 +18,7 @@ def split_dataset_by_node(dataset: DatasetType, rank: int, world_size: int) -> D
 
     For iterable datasets:
 
-    If the dataset has a number of shards that is a factor of `world_size` (i.e. if `dataset.n_shards % world_size == 0`),
+    If the dataset has a number of shards that is a factor of `world_size` (i.e. if `dataset.num_shards % world_size == 0`),
     then the shards are evenly assigned across the nodes, which is the most optimized.
     Otherwise, each node keeps 1 example out of `world_size`, skipping the other examples.