Add docstring for scatter (#1189)

* Add docstring for scatter

* docs nits

---------

Co-authored-by: Awni Hannun <awni@apple.com>

mlx/ops.h

@@ -927,7 +927,104 @@ array take_along_axis(
     int axis,
     StreamOrDevice s = {});
 
-/** Scatter updates to given linear indices */
+/** Scatter updates to the given indices.
+ *
+ * The parameters ``indices`` and ``axes`` determine the locations of ``a``
+ * that are updated with the values in ``updates``. Assuming 1-d ``indices``
+ * for simplicity, ``indices[i]`` are the indices on axis ``axes[i]`` to which
+ * the values in ``updates`` will be applied. Note each array in
+ * ``indices`` is assigned to a corresponding axis and hence ``indices.size() ==
+ * axes.size()``. If an index/axis pair is not provided then indices along that
+ * axis are assumed to be zero.
+ *
+ * Note the rank of ``updates`` must be equal to the sum of the rank of the
+ * broadcasted ``indices`` and the rank of ``a``. In other words, assuming the
+ * arrays in ``indices`` have the same shape, ``updates.ndim() ==
+ * indices[0].ndim() + a.ndim()``. The leading dimensions of ``updates``
+ * correspond to the indices, and the remaining ``a.ndim()`` dimensions are the
+ * values that will be applied to the given location in ``a``.
+ *
+ * For example:
+ *
+ * @code
+ * auto in = zeros({4, 4}, float32);
+ * auto indices = array({2});
+ * auto updates = reshape(arange(1, 3, float32), {1, 1, 2});
+ * std::vector<int> axes{0};
+ *
+ * auto out = scatter(in, {indices}, updates, axes);
+ * @endcode
+ *
+ * will produce:
+ *
+ * @code
+ * array([[0, 0, 0, 0],
+ *        [0, 0, 0, 0],
+ *        [1, 2, 0, 0],
+ *        [0, 0, 0, 0]], dtype=float32)
+ * @endcode
+ *
+ * This scatters the two-element row vector ``[1, 2]`` starting at the ``(2,
+ * 0)`` position of ``a``.
+ *
+ * Adding another element to ``indices`` will scatter into another location of
+ * ``a``. We also have to add an another update for the new index:
+ *
+ * @code
+ * auto in = zeros({4, 4}, float32);
+ * auto indices = array({2, 0});
+ * auto updates = reshape(arange(1, 5, float32), {2, 1, 2});
+ * std::vector<int> axes{0};
+ *
+ * auto out = scatter(in, {indices}, updates, axes):
+ * @endcode
+ *
+ * will produce:
+ *
+ * @code
+ * array([[3, 4, 0, 0],
+ *        [0, 0, 0, 0],
+ *        [1, 2, 0, 0],
+ *        [0, 0, 0, 0]], dtype=float32)
+ * @endcode
+ *
+ * To control the scatter location on an additional axis, add another index
+ * array to ``indices`` and another axis to ``axes``:
+ *
+ * @code
+ * auto in = zeros({4, 4}, float32);
+ * auto indices = std::vector{array({2, 0}), array({1, 2})};
+ * auto updates = reshape(arange(1, 5, float32), {2, 1, 2});
+ * std::vector<int> axes{0, 1};
+ *
+ * auto out = scatter(in, indices, updates, axes);
+ * @endcode
+ *
+ * will produce:
+ *
+ * @code
+ * array([[0, 0, 3, 4],
+ *       [0, 0, 0, 0],
+ *       [0, 1, 2, 0],
+ *       [0, 0, 0, 0]], dtype=float32)
+ * @endcode
+ *
+ * Items in indices are broadcasted together. This means:
+ *
+ * @code
+ * auto indices = std::vector{array({2, 0}), array({1})};
+ * @endcode
+ *
+ * is equivalent to:
+ *
+ * @code
+ * auto indices = std::vector{array({2, 0}), array({1, 1})};
+ * @endcode
+ *
+ * Note, ``scatter`` does not perform bounds checking on the indices and
+ * updates.  Out-of-bounds accesses on ``a`` are undefined and typically result
+ * in unintended or invalid memory writes.
+ */
 array scatter(
     const array& a,
     const std::vector<array>& indices,