[APFloat] Add E4M3B11FNUZ

X. Sun et al. (https://dl.acm.org/doi/10.5555/3454287.3454728) published
a paper showing that an FP format with 4 bits of exponent, 3 bits of
significand and an exponent bias of 11 would work quite well for ML
applications.

Google hardware supports a variant of this format where 0x80 is used to
represent NaN, as in the Float8E4M3FNUZ format. Just like the
Float8E4M3FNUZ format, this format does not support -0 and values which
would map to it will become +0.

This format is proposed for inclusion in OpenXLA's StableHLO dialect: https://github.com/openxla/stablehlo/pull/1308

As part of inclusion in that dialect, APFloat needs to know how to
handle this format.

Differential Revision: https://reviews.llvm.org/D146441

mlir/python/mlir/_mlir_libs/_mlir/ir.pyi

@@ -53,6 +53,7 @@ __all__ = [
     "Float8E4M3FNType",
     "Float8E5M2Type",
     "Float8E4M3FNUZType",
+    "Float8E4M3B11FNUZType",
     "Float8E5M2FNUZType",
     "F16Type",
     "F32Type",
@@ -602,6 +603,13 @@ class Float8E4M3FNUZType(Type):
     @staticmethod
     def isinstance(arg: Any) -> bool: ...
 
+class Float8E4M3B11FNUZType(Type):
+    def __init__(self, cast_from_type: Type) -> None: ...
+    @staticmethod
+    def get(*args, **kwargs) -> Float8E4M3B11FNUZType: ...
+    @staticmethod
+    def isinstance(arg: Any) -> bool: ...
+
 class Float8E5M2FNUZType(Type):
     def __init__(self, cast_from_type: Type) -> None: ...
     @staticmethod