[Lang] Add cast to field.fill() interface (taichi-dev#7899)

Issue: #taichi-dev#7678

There's one potential hazard: if the user mistakenly filled a ti.field
with out-of-scope values (like filling a ti.field(ti.u8) with > 255),
it's just gonna overflow silently.

python/taichi/_kernels.py

@@ -19,9 +19,10 @@
 
 # A set of helper (meta)functions
 @kernel
-def fill_tensor(tensor: template(), val: template()):
-    for I in grouped(tensor):
-        tensor[I] = val
+def fill_field(field: template(), val: template()):
+    value = ops.cast(val, field.dtype)
+    for I in grouped(field):
+        field[I] = value
 
 
 @kernel

python/taichi/lang/field.py

@@ -282,9 +282,9 @@ def __init__(self, var):
     def fill(self, val):
         """Fills this scalar field with a specified value."""
         if in_python_scope():
-            from taichi._kernels import fill_tensor  # pylint: disable=C0415
+            from taichi._kernels import fill_field  # pylint: disable=C0415
 
-            fill_tensor(self, val)
+            fill_field(self, val)
         else:
             from taichi._funcs import field_fill_taichi_scope  # pylint: disable=C0415