[STYLE] run autopep8 and isort (#421)

Run:
```
isort ./python
autopep8 -i --ignore E501,E701,E731 $(find ./python/ -name '*.py')
```
with an `.isort.cfg` and then clean up a few warts. This PR should be a no-op; the idea is that this is all boring whitespace changes, and any config file changes will be in a different change to make it easier to review.

python/tutorials/01-vector-add.py

@@ -12,8 +12,8 @@ In this tutorial, you will write a simple vector addition using Triton and learn
 # Compute Kernel
 # --------------------------
 
-from triton.language.core import constexpr
 import torch
+
 import triton
 import triton.language as tl
 
@@ -38,7 +38,7 @@ def add_kernel(
     offsets = block_start + tl.arange(0, BLOCK_SIZE)
     # Create a mask to guard memory operations against out-of-bounds accesses
     mask = offsets < n_elements
-    # Load x and y from DRAM, masking out any extra elements in case 
+    # Load x and y from DRAM, masking out any extra elements in case
     # the input is not a multiple of the block size
     x = tl.load(x_ptr + offsets, mask=mask)
     y = tl.load(y_ptr + offsets, mask=mask)