PR #19096: Add F4E2M1FN and F8E8M0FNU types

Imported from GitHub PR openxla/xla#19096

This PR adds F4E2M1FN primitive type (4-bit float with 2 bits exponent and 1 bit mantissa), F8E8M0FNU primitive type (8-bit float with 8 bits exponent, no mantissa and no sign) and enables loads/stores in the same way S4/U4 type is implemented.

This will enable using microscaling (MX) formats ([RFC](openxla/xla#18085)), such as MXFP4.

```c...

PiperOrigin-RevId: 709153611

tsl/platform/BUILD

@@ -985,7 +985,6 @@ cc_library(
     deps = [
         "@ml_dtypes//:float8",
         "@ml_dtypes//:intn",
-        "@ml_dtypes//:mxfloat",
     ],
 )
 

tsl/platform/ml_dtypes.h

@@ -18,18 +18,15 @@ limitations under the License.
 
 #include "ml_dtypes/include/float8.h"  // from @ml_dtypes
 #include "ml_dtypes/include/intn.h"  // from @ml_dtypes
-#include "ml_dtypes/include/mxfloat.h"  // from @ml_dtypes
 
 namespace tsl {
-using float4_e2m1fn = ::ml_dtypes::float4_e2m1fn;
 using float8_e3m4 = ::ml_dtypes::float8_e3m4;
 using float8_e4m3 = ::ml_dtypes::float8_e4m3;
 using float8_e4m3fn = ::ml_dtypes::float8_e4m3fn;
 using float8_e4m3fnuz = ::ml_dtypes::float8_e4m3fnuz;
 using float8_e4m3b11fnuz = ::ml_dtypes::float8_e4m3b11fnuz;
 using float8_e5m2 = ::ml_dtypes::float8_e5m2;
 using float8_e5m2fnuz = ::ml_dtypes::float8_e5m2fnuz;
-using float8_e8m0fnu = ::ml_dtypes::float8_e8m0fnu;
 
 using int1 = ::ml_dtypes::int1;
 using uint1 = ::ml_dtypes::uint1;