nanogpt: fix multiprocessing in load_dataset on os x

The issue seems to be that _fixup_main_from_path in multiprocessing
module in python is unable to find entry point, thus, adding
```
if __name__ == '__main__'
```

data/openwebtext/prepare.py

@@ -16,64 +16,65 @@ num_proc = 8
 # it is better than 1 usually though
 num_proc_load_dataset = num_proc
 
-# takes 54GB in huggingface .cache dir, about 8M documents (8,013,769)
-dataset = load_dataset("openwebtext", num_proc=num_proc_load_dataset)
+if __name__ == '__main__':
+    # takes 54GB in huggingface .cache dir, about 8M documents (8,013,769)
+    dataset = load_dataset("openwebtext", num_proc=num_proc_load_dataset)
 
-# owt by default only contains the 'train' split, so create a test split
-split_dataset = dataset["train"].train_test_split(test_size=0.0005, seed=2357, shuffle=True)
-split_dataset['val'] = split_dataset.pop('test') # rename the test split to val
+    # owt by default only contains the 'train' split, so create a test split
+    split_dataset = dataset["train"].train_test_split(test_size=0.0005, seed=2357, shuffle=True)
+    split_dataset['val'] = split_dataset.pop('test') # rename the test split to val
 
-# this results in:
-# >>> split_dataset
-# DatasetDict({
-#     train: Dataset({
-#         features: ['text'],
-#         num_rows: 8009762
-#     })
-#     val: Dataset({
-#         features: ['text'],
-#         num_rows: 4007
-#     })
-# })
+    # this results in:
+    # >>> split_dataset
+    # DatasetDict({
+    #     train: Dataset({
+    #         features: ['text'],
+    #         num_rows: 8009762
+    #     })
+    #     val: Dataset({
+    #         features: ['text'],
+    #         num_rows: 4007
+    #     })
+    # })
 
-# we now want to tokenize the dataset. first define the encoding function (gpt2 bpe)
-enc = tiktoken.get_encoding("gpt2")
-def process(example):
-    ids = enc.encode_ordinary(example['text']) # encode_ordinary ignores any special tokens
-    ids.append(enc.eot_token) # add the end of text token, e.g. 50256 for gpt2 bpe
-    # note: I think eot should be prepended not appended... hmm. it's called "eot" though...
-    out = {'ids': ids, 'len': len(ids)}
-    return out
+    # we now want to tokenize the dataset. first define the encoding function (gpt2 bpe)
+    enc = tiktoken.get_encoding("gpt2")
+    def process(example):
+        ids = enc.encode_ordinary(example['text']) # encode_ordinary ignores any special tokens
+        ids.append(enc.eot_token) # add the end of text token, e.g. 50256 for gpt2 bpe
+        # note: I think eot should be prepended not appended... hmm. it's called "eot" though...
+        out = {'ids': ids, 'len': len(ids)}
+        return out
 
-# tokenize the dataset
-tokenized = split_dataset.map(
-    process,
-    remove_columns=['text'],
-    desc="tokenizing the splits",
-    num_proc=num_proc,
-)
+    # tokenize the dataset
+    tokenized = split_dataset.map(
+        process,
+        remove_columns=['text'],
+        desc="tokenizing the splits",
+        num_proc=num_proc,
+    )
 
-# concatenate all the ids in each dataset into one large file we can use for training
-for split, dset in tokenized.items():
-    arr_len = np.sum(dset['len'], dtype=np.uint64)
-    filename = os.path.join(os.path.dirname(__file__), f'{split}.bin')
-    dtype = np.uint16 # (can do since enc.max_token_value == 50256 is < 2**16)
-    arr = np.memmap(filename, dtype=dtype, mode='w+', shape=(arr_len,))
-    total_batches = 1024
+    # concatenate all the ids in each dataset into one large file we can use for training
+    for split, dset in tokenized.items():
+        arr_len = np.sum(dset['len'], dtype=np.uint64)
+        filename = os.path.join(os.path.dirname(__file__), f'{split}.bin')
+        dtype = np.uint16 # (can do since enc.max_token_value == 50256 is < 2**16)
+        arr = np.memmap(filename, dtype=dtype, mode='w+', shape=(arr_len,))
+        total_batches = 1024
 
-    idx = 0
-    for batch_idx in tqdm(range(total_batches), desc=f'writing {filename}'):
-        # Batch together samples for faster write
-        batch = dset.shard(num_shards=total_batches, index=batch_idx, contiguous=True).with_format('numpy')
-        arr_batch = np.concatenate(batch['ids'])
-        # Write into mmap
-        arr[idx : idx + len(arr_batch)] = arr_batch
-        idx += len(arr_batch)
-    arr.flush()
+        idx = 0
+        for batch_idx in tqdm(range(total_batches), desc=f'writing {filename}'):
+            # Batch together samples for faster write
+            batch = dset.shard(num_shards=total_batches, index=batch_idx, contiguous=True).with_format('numpy')
+            arr_batch = np.concatenate(batch['ids'])
+            # Write into mmap
+            arr[idx : idx + len(arr_batch)] = arr_batch
+            idx += len(arr_batch)
+        arr.flush()
 
-# train.bin is ~17GB, val.bin ~8.5MB
-# train has ~9B tokens (9,035,582,198)
-# val has ~4M tokens (4,434,897)
+    # train.bin is ~17GB, val.bin ~8.5MB
+    # train has ~9B tokens (9,035,582,198)
+    # val has ~4M tokens (4,434,897)
 
-# to read the bin files later, e.g. with numpy:
-# m = np.memmap('train.bin', dtype=np.uint16, mode='r')
+    # to read the bin files later, e.g. with numpy:
+    # m = np.memmap('train.bin', dtype=np.uint16, mode='r')