decouple IDX format from raw tensor

[lgarithm]

The IDX format only defined encodings for: u8, i8, i16, i32, f32, f64.
The new encoding format should support more scalar data types, and should be extendable.

[lgarithm]

The new encoding scheme should support all scalar data types given by the combination:

(category, bytes, bits-per-byte, variant)

where

category := unsigned | signed | floating point
bytes := 1 | 2 | 4 | 8 or other values if necessary
bits-per-byte := 8 or other values if necessary
variant := 0 | or other values if present

the variant component is useful to distinguish IEEE 754 float16 and bfloat16.

string type can be also encoded by the encoding scheme by adding a new category.

type	category	bytes	bits-per-byte	variant
u8	unsigned	1	8	0
u16	unsigned	2	8	0
u32	unsigned	4	8	0
u64	unsigned	8	8	0
u128	unsigned	16	8	0
i8	signed	1	8	0
i16	signed	2	8	0
i32	signed	4	8	0
i64	signed	8	8	0
i128	signed	16	8	0
f16 (IEEE 754)	floating point	2	8	0
f16 (bfloat16)	floating point	2	8	1
f32(IEEE 754)	floating point	4	8	0
f64(IEEE 754)	floating point	8	8	0
f128 ?	floating point	16	8	?
basic_string<char>	str	sizeof(basic_string<char>)	8	?

Binary represent of the encoding:

|01234567|01234567|
|AAABBBBB|CCCCDDDD|

component	bits	value range	common values
category	3	0 ~ 7	1,2,3,4, ...
bytes	5	0 ~ 31	1,2,4,8,12,16
bits-per-byte	4	0 ~ 15	8
variant	4	0 ~ 15	0,1

[lgarithm]

template <typename R>
constexpr uint32_t basic_scalar_category()
{
    if (std::is_integral<R>::value) {
        if (std::is_unsigned<R>::value) { return 0; }
        if (std::is_signed<R>::value) { return 1; }
    }
    if (std::is_floating_point<R>::value) { return 2; }
    return -1;
}

// template <typename R, typename V = uint32_t>
// struct basic_scalar_category;

template <typename R, typename V = uint32_t>
class basic_scalar_encoding;

template <typename R, typename V>
class basic_scalar_encoding
{
    static constexpr V category = basic_scalar_category<R>();
    static constexpr V byte_num = sizeof(R);
    static constexpr V byte_size = CHAR_BIT;

  public:
    static constexpr V value = (category << 16) | (byte_num << 8) | byte_size;
};