fix np.memmap memory leak

nn.memmap doesn't free memory that it accesses. Thus, the entire dataset gets stored in RAM as the dataset has been fully accessed. The simplest workaround on stackoverflow is to just recreate the memmap for each batch. The extra overhead is negligible.

https://stackoverflow.com/questions/45132940/numpy-memmap-memory-usage-want-to-iterate-once/61472122#61472122

train.py

@@ -113,10 +113,13 @@
 
 # poor man's data loader
 data_dir = os.path.join('data', dataset)
-train_data = np.memmap(os.path.join(data_dir, 'train.bin'), dtype=np.uint16, mode='r')
-val_data = np.memmap(os.path.join(data_dir, 'val.bin'), dtype=np.uint16, mode='r')
 def get_batch(split):
-    data = train_data if split == 'train' else val_data
+    # We recreate np.memmap every batch to avoid a memory leak, as per
+    # https://stackoverflow.com/questions/45132940/numpy-memmap-memory-usage-want-to-iterate-once/61472122#61472122
+    if split == 'train':
+        data = np.memmap(os.path.join(data_dir, 'train.bin'), dtype=np.uint16, mode='r')
+    else:
+        data = np.memmap(os.path.join(data_dir, 'val.bin'), dtype=np.uint16, mode='r')
     ix = torch.randint(len(data) - block_size, (batch_size,))
     x = torch.stack([torch.from_numpy((data[i:i+block_size]).astype(np.int64)) for i in ix])
     y = torch.stack([torch.from_numpy((data[i+1:i+1+block_size]).astype(np.int64)) for i in ix])