replace np.frombuffer with torch.frombuffer in MNIST prototype (#4651)

* replace np.frombuffer with torch.frombuffer in MNIST prototype

* cleanup

* appease mypy

* more cleanup

* clarify inplace offset

* fix num bytes for floating point data

torchvision/prototype/datasets/_builtin/mnist.py

@@ -5,9 +5,9 @@ import io
 import operator
 import pathlib
 import string
-from typing import Any, Callable, Dict, Iterator, List, Optional, Tuple, cast, Union
+import sys
+from typing import Any, Callable, Dict, Iterator, List, Optional, Tuple, cast
 
-import numpy as np
 import torch
 from torchdata.datapipes.iter import (
     IterDataPipe,
@@ -38,14 +38,14 @@ __all__ = ["MNIST", "FashionMNIST", "KMNIST", "EMNIST", "QMNIST"]
 prod = functools.partial(functools.reduce, operator.mul)
 
 
-class MNISTFileReader(IterDataPipe[np.ndarray]):
+class MNISTFileReader(IterDataPipe[torch.Tensor]):
     _DTYPE_MAP = {
-        8: "u1",  # uint8
-        9: "i1",  # int8
-        11: "i2",  # int16
-        12: "i4",  # int32
-        13: "f4",  # float32
-        14: "f8",  # float64
+        8: torch.uint8,
+        9: torch.int8,
+        11: torch.int16,
+        12: torch.int32,
+        13: torch.float32,
+        14: torch.float64,
     }
 
     def __init__(
@@ -59,18 +59,20 @@ class MNISTFileReader(IterDataPipe[np.ndarray]):
     def _decode(bytes: bytes) -> int:
         return int(codecs.encode(bytes, "hex"), 16)
 
-    def __iter__(self) -> Iterator[np.ndarray]:
+    def __iter__(self) -> Iterator[torch.Tensor]:
         for _, file in self.datapipe:
             magic = self._decode(file.read(4))
-            dtype_type = self._DTYPE_MAP[magic // 256]
+            dtype = self._DTYPE_MAP[magic // 256]
             ndim = magic % 256 - 1
 
             num_samples = self._decode(file.read(4))
             shape = [self._decode(file.read(4)) for _ in range(ndim)]
 
-            in_dtype = np.dtype(f">{dtype_type}")
-            out_dtype = np.dtype(dtype_type)
-            chunk_size = (cast(int, prod(shape)) if shape else 1) * in_dtype.itemsize
+            num_bytes_per_value = (torch.finfo if dtype.is_floating_point else torch.iinfo)(dtype).bits // 8
+            # The MNIST format uses the big endian byte order. If the system uses little endian byte order by default,
+            # we need to reverse the bytes before we can read them with torch.frombuffer().
+            needs_byte_reversal = sys.byteorder == "little" and num_bytes_per_value > 1
+            chunk_size = (cast(int, prod(shape)) if shape else 1) * num_bytes_per_value
 
             start = self.start or 0
             stop = self.stop or num_samples
@@ -78,11 +80,15 @@ class MNISTFileReader(IterDataPipe[np.ndarray]):
             file.seek(start * chunk_size, 1)
             for _ in range(stop - start):
                 chunk = file.read(chunk_size)
-                yield np.frombuffer(chunk, dtype=in_dtype).astype(out_dtype).reshape(shape)
+                if not needs_byte_reversal:
+                    yield torch.frombuffer(chunk, dtype=dtype).reshape(shape)
+
+                chunk = bytearray(chunk)
+                chunk.reverse()
+                yield torch.frombuffer(chunk, dtype=dtype).flip(0).reshape(shape)
 
 
 class _MNISTBase(Dataset):
-    _FORMAT = "png"
     _URL_BASE: str
 
     @abc.abstractmethod
@@ -105,24 +111,23 @@ class _MNISTBase(Dataset):
 
     def _collate_and_decode(
         self,
-        data: Tuple[np.ndarray, np.ndarray],
+        data: Tuple[torch.Tensor, torch.Tensor],
         *,
         config: DatasetConfig,
         decoder: Optional[Callable[[io.IOBase], torch.Tensor]],
     ) -> Dict[str, Any]:
-        image_array, label_array = data
+        image, label = data
 
-        image: Union[torch.Tensor, io.BytesIO]
         if decoder is raw:
-            image = torch.from_numpy(image_array)
+            image = image.unsqueeze(0)
         else:
-            image_buffer = image_buffer_from_array(image_array)
-            image = decoder(image_buffer) if decoder else image_buffer
+            image_buffer = image_buffer_from_array(image.numpy())
+            image = decoder(image_buffer) if decoder else image_buffer  # type: ignore[assignment]
 
-        label = torch.tensor(label_array, dtype=torch.int64)
         category = self.info.categories[int(label)]
+        label = label.to(torch.int64)
 
-        return dict(image=image, label=label, category=category)
+        return dict(image=image, category=category, label=label)
 
     def _make_datapipe(
         self,
@@ -293,12 +298,11 @@ class EMNIST(_MNISTBase):
 
     def _collate_and_decode(
         self,
-        data: Tuple[np.ndarray, np.ndarray],
+        data: Tuple[torch.Tensor, torch.Tensor],
         *,
         config: DatasetConfig,
         decoder: Optional[Callable[[io.IOBase], torch.Tensor]],
     ) -> Dict[str, Any]:
-        image_array, label_array = data
         # In these two splits, some lowercase letters are merged into their uppercase ones (see Fig 2. in the paper).
         # That means for example that there is 'D', 'd', and 'C', but not 'c'. Since the labels are nevertheless dense,
         # i.e. no gaps between 0 and 46 for 47 total classes, we need to add an offset to create this gaps. For example,
@@ -308,8 +312,8 @@ class EMNIST(_MNISTBase):
         # index 39 (10 digits + 26 uppercase letters + 4th lower case letter - 1 for zero indexing)
         # in self.categories. Thus, we need to add 1 to the label to correct this.
         if config.image_set in ("Balanced", "By_Merge"):
-            label_array += np.array(self._LABEL_OFFSETS.get(int(label_array), 0), dtype=label_array.dtype)
-        return super()._collate_and_decode((image_array, label_array), config=config, decoder=decoder)
+            data[1] += self._LABEL_OFFSETS.get(int(data[1]), 0)
+        return super()._collate_and_decode(data, config=config, decoder=decoder)
 
     def _make_datapipe(
         self,
@@ -379,22 +383,22 @@ class QMNIST(_MNISTBase):
 
     def _collate_and_decode(
         self,
-        data: Tuple[np.ndarray, np.ndarray],
+        data: Tuple[torch.Tensor, torch.Tensor],
         *,
         config: DatasetConfig,
         decoder: Optional[Callable[[io.IOBase], torch.Tensor]],
     ) -> Dict[str, Any]:
-        image_array, label_array = data
-        label_parts = label_array.tolist()
-        sample = super()._collate_and_decode((image_array, label_parts[0]), config=config, decoder=decoder)
+        image, ann = data
+        label, *extra_anns = ann
+        sample = super()._collate_and_decode((image, label), config=config, decoder=decoder)
 
         sample.update(
             dict(
                 zip(
                     ("nist_hsf_series", "nist_writer_id", "digit_index", "nist_label", "global_digit_index"),
-                    label_parts[1:6],
+                    [int(value) for value in extra_anns[:5]],
                 )
             )
         )
-        sample.update(dict(zip(("duplicate", "unused"), [bool(value) for value in label_parts[-2:]])))
+        sample.update(dict(zip(("duplicate", "unused"), [bool(value) for value in extra_anns[-2:]])))
         return sample