ENH: add create_diagonal

Co-authored-by: Jake Bowhay <[email protected]>

src/array_api_extra/_funcs.py

@@ -140,6 +140,14 @@ def cov(m: Array, /, *, xp: ModuleType) -> Array:
     return xp.squeeze(c, axis=axes)
 
 
+def create_diagonal(x: Array, /, *, offset: int = 0, xp: ModuleType) -> Array:
+    n = x.shape[0] + abs(offset)
+    diag = xp.zeros(n**2, dtype=x.dtype)
+    i = offset if offset >= 0 else abs(offset) * n
+    diag[i : min(n * (n - offset), diag.shape[0]) : n + 1] = x
+    return xp.reshape(diag, (n, n))
+
+
 def _mean(
     x: Array,
     /,